Data transmission method and data transmission apparatus

ABSTRACT

A data transmission apparatus including a receiving unit for receiving transmitted packets; a priority decision unit; a retransmission packet storage unit; a retransmission instruction receiving unit for receiving a retransmission request from a terminal at the receiving end; a retransmission decision unit; a transmission queue management unit; and a transmission unit.

This is a Rule 1.53(b) Divisional application of Ser. No. 10/227,456filed Aug. 26, 2002 now U.S. Pat. No. 6,684,354, which is a Rule 1.53(b)Continuation application of Ser. No. 09/450,590, filed Nov. 30, 1999 nowU.S. Pat. No. 6,587,985.

FIELD OF THE INVENTION

The present invention relates to data transmission methods datatransmission apparatuses, data receiving apparatuses, and a packet datastructure and, more particularly, to a process of performing datatransmission in packet units between a distribution server and aterminal while successively reproducing the data of received packets atthe terminal.

BACKGROUND OF THE INVENTION

For transmission of video (audio and video) data on the Internet, adownload type transmission method and a stream type transmission methodare currently employed.

In the download type transmission method, a video file transmitted froma distribution server is once copied at the terminal and, thereafter,data of the video file (video data) is reproduced. So, the terminalcannot perform data reproduction until the file transmission iscompleted. Therefore, the download type transmission method is notsuitable for long-hours of reproduction of video data or the like.

On the other hand, in the stream type transmission method, while videodata or the like is transmitted from a distribution server to aterminal, received data is reproduced at the terminal.

Recently, a stream type transmission method using a protocol called RTP(Real-time Transport Protocol) which is defined in IETE RFC 1889, hasmainly been used.

FIG. 28(a) is a diagram for explaining a video data transmission methodaccording to the RTP.

With reference to FIG. 28(a), a distribution server (transmitting end)and a terminal (receiving end) such as a personal computer are connectedby a cable in the Internet, such as modem, ISDN, or LAN, andtransmission of video data is carried out between the server and theterminal by using the RTP.

In the data transmission according to the RTP, processes for therespective packets are synchronized between the transmitting end and thereceiving end by using time stamps as time information, and asynchronous(late arrival) packets and packets affected by transmission errors arediscarded at the receiving end. Further, discarded or lost packets aredetected at the receiving end, according to the absence of sequencenumbers give to these packets.

On the other hand, Internet applications utilizing mobile phones, forexample, mail access and text information service, are advancing now,and infrastructure for high-speed radio data communication (high-speedpacket communication of ˜384 kbps) is under preparation for practicaluse of the third generation mobile communication (W-CDMA: Wide band-CodeDivision Multiple Access).

FIG. 28(b), is a diagram illustrating a communication network for theabove-described W-CDMA visual terminal.

Such communication network includes a radio transmission section. Forexample, when performing data transmission between a video distributionserver and a visual terminal through a relay server, the section betweenthe distribution server and the relay server is a cable transmissionsection by the Internet, but the section between the relay server andthe visual terminal is a radio transmission section by a mobile phonenetwork such as the W-CDMA.

However, the bit error rate in the radio transmission section is 10⁻³while the bit error rate in the cable transmission section is 10⁻⁵˜10⁻⁷,and the radio transmission quality becomes a problem in the RTP typedata transmission method in which the reproduction quality depends onthe end-to-end (server-to-terminal) transmission quality.

SUMMARY OF THE INVENTION

The present invention is made to solve the above-described problems andhas for its object to provide a data transmission method, a datatransmission apparatus, and a data receiving apparatus, which canimprove the transmission quality in a radio section in real-timetransmission.

Other objects and advantages of the invention will become apparent fromthe detailed description that follows. The detailed description andspecific embodiments described are provided only for illustration sincevarious additions and modifications within the scope of the inventionwill be apparent to those of skill in the art from the detaileddescription.

According to a first aspect of the present invention, there is provideda data transmission method for performing continuous data transmissionfrom the transmitting end to the receiving end in units of packets, eachpacket having additional information relating to its sequence number,priority, and data reproduction time at the receiving end, whilesuccessively reproducing data of packets received at the receiving end.This method comprising the steps of: at the transmitting end, givingpriority information to each packet to be transmitted; and storing, asretransmission data, only data of packets the priorities of which areequal to or higher than a predetermined value, in a retransmissionbuffer; at the receiving end, when a transmission error is detected,detecting the priority information of an error packet; and when thedetected priority is equal to or higher than the predetermined value,outputting a retransmission request for the error packet to thetransmitting end by indicating the sequence number of this error packet;at the transmitting end, only when the data of the packet having thesequence number which is indicated by the retransmission request fromthe receiving end is stored in the retransmission buffer, retransmittingthe data of this packet to the receiving end; and discarding the datastored in the retransmission buffer in order starting from a packetwhich cannot be in time for data reproduction at the receiving end.Therefore, the transmission quality in a radio section in real-timetransmission is improved and, further, the number of retransmissiontimes is reduced.

According to a second aspect of the present invention, in the datatransmission method of the first aspect, when the retransmission bufferis filled up to its capacity, an updating process is performed, in whichthe retransmission data are retained while the data stored in theretransmission buffer are discarded in order, starting from a packet ofthe earliest reproduction time, on the basis of the reproduction time ofeach packet stored in the retransmission buffer. Therefore, thetransmission quality in a radio section in real-time transmission isimproved and, further, the number of retransmission times is reduced.

According to a third aspect of the present invention, in the datatransmission method of the first aspect, when the data transmitted fromthe transmitting end to the receiving end is video data based on MPEG, apacket which contains data corresponding to frames coded by utilizingintra-frame correlation is regarded as a packet having a high priority.Therefore, the transmission quality in a radio section in real-timetransmission is improved and, further, the number of retransmissiontimes is reduced.

According to a fourth aspect of the present invention, in the datatransmission method of the first aspect, at the transmitting end, theadditional, information relating to the sequence number and the priorityof a predetermined packet is also embedded in a subsequent packet to betransmitted after the predetermined packet; and at the receiving end, inthe case where a transmission error has occurred in the predeterminedpacket an the additional information of the predetermined packet has anerror, a retransmission request for the predetermined packet as an errorpacket is made on the basis of the additional information of thepredetermined packet which is embedded in the subsequent packet, whenthe subsequent packet transmitted after the predetermined packet isreceived. Therefore, the transmission quality in a radio section inreal-time transmission is improved and, further, the number ofretransmission times is reduced.

According to a fifth aspect of the present invention, in the datatransmission method of the fourth aspect, at the transmitting end, theprocess of embedding the sequence number of a predetermined highpriority packet in a subsequent packet which follows the predeterminedhigh priority packet is continuously performed until a high prioritypacket next to the predetermined high priority packet is transmitted;and at the receiving end, the sequence number of another packet which isembedded in the received packet is extracted, and when a transmissionerror has occurred in the packet of the extracted sequence number, aretransmission request for this error packet is made by indicating thesequence number of this packet. Therefore, the transmission quality in aradio section in real-time transmission is improved and, further, thenumber of retransmission times is reduced.

According to a sixth aspect of the present invention, there is provideda data transmission apparatus for relaying data which are successivelytransmitted from the transmitting end in units of packets, each packethaving additional information relating to its sequence number, priority,data reproduction time at the receiving end. This apparatus comprises: areceiving unit for receiving the packets transmitted from thetransmitting end; a priority decision unit for deciding the priority ofeach of the received packets; a retransmission packet storage unit forstoring packets the priorities of which are equal to or higher than apredetermined value, as retransmission packets, on the basis of thepriority of each packet decided by the priority decision unit; aretransmission instruction receiving unit for receiving a retransmissionrequest from a terminal at the receiving end; a retransmission decisionunit for deciding whether retransmission of the packet for which theretransmission request has been made should be performed or not, on thebasis of the retransmission request and the storage status of theretransmission packets in the retransmission packet storage unit;transmission queue management unit for setting the transmission order ofthe received packets and the packets which have been decided as packetsto be retransmitted, on the basis of the additional information; and atransmission unit for transmitting the data of these packets in thetransmission order set by the management unit. Therefore, only the errorpackets the priorities of which are equal to or higher than apredetermined value can be retransmitted, whereby the transmissionquality of radio section in real-time transmission is improved and,further, the number of retransmission times is reduced.

According to a seventh aspect of the present invention, there isprovided a data receiving apparatus for receiving data which aretransmitted from the transmitting end in units of packets, each packethaving additional information relating to its sequence number, priority,and data reproduction time at the receiving end, and successivelyreproducing the data for each packet. This apparatus comprises: areceiving unit for receiving the packets transmitted from thetransmitting end; an error packet detection unit for detecting errorpackets in which error have occurred during transmission, and outputtingnormal packets which have been transmitted without transmission errors,on the basis of the data of the received packets; a packet prioritydecision unit for receiving the output from the error packet detectionunit, and deciding error packets the priorities of which are equal to orhigher than a predetermined value; and a retransmission instructionoutput, unit for outputting a retransmission, request for each of theerror packets the priorities of which are decided as being equal to orhigher than the predetermined value, to the transmitting end, byindicating the sequence number of this error packet. Therefore, at thereceiving end, a retransmission request to the transmitting end is madeonly for the error packet the priority of which is equal to or higherthan a predetermined value, whereby the transmission quality in a radiosection in real-time transmission is improved and, further, the numberof retransmission times is reduced.

According to an eighth aspect of the present invention, there isprovided a data transmission method in which data transmission from thetransmitting end to the receiving end is continuously performed in unitsof packets, each packet having additional information relating to itssequence number, priority, and data reproduction time at the receivingend, while successively reproducing data of packets which have arrivedat the receiving end and, at this time, only packets which can be intime for data reproduction at the receiving end are retransmitted. Thismethod comprises: at the transmitting end, giving a data reproductiontime at the receiving end to each packet to be transmitted; and storing,as retransmission data, only data of packets the priorities of which areequal to or higher than a predetermined value, in a retransmissionbuffer; at the receiving end, when a transmission error is detected,detecting the reproduction time for an error packet and the arrival timeof the error packet, and deciding an arrival time limit in accordancewith the reproduction time; and when the error packet has arrived beforethe arrival time limit, outputting a retransmission request for theerror packet to the transmitting end by indicating the sequence numberof this error packet; at the transmitting end, when the data of thepacket having the sequence number indicated by the retransmissionrequest from the receiving end is stored in the retransmission buffer,retransmitting data of the packet the transmission time of which doesnot pass the reproduction time, to the receiving end, while discardingdata of the packet the transmission time of which has passed thereproduction time; and discarding the data stored in the retransmissionbuffer in order starting from a packet which cannot be in time for datareproduction at the receiving end. Therefore, the transmission qualityin a radio section in real-time transmission is improved and, further,the number of retransmission times is reduced.

According to a ninth aspect of the present invention, in the datatransmission method of the eighth aspect, when the retransmission bufferis filled up to its capacity, an updating processes is performed, inwhich the retransmission data are retained while the data stored in theretransmission buffer are discarded in order, starting from a packet ofthe earliest reproduction time, on the basis of the reproduction time ofeach packet stored in the retransmission buffer. Therefore, thetransmission quality in a radio section in real-time transmission isimproved and, further, the number of retransmission times is reduced.

According to a tenth aspect of the present invention, in the datatransmission method of the eighth aspect, the arrival time limit isdecided based on at least one of the allowable packet delay time decidedat the receiving end, and the packet transmission delay time between thetransmitting end and the receiving end. Therefore, the transmissionquality in a radio section in real-time transmission is improved and,further, the number of retransmission times is reduced.

According to an eleventh aspect of the present invention, in the datatransmission method of the eighth aspect, at the transmitting end,additional information relating to the sequence number and thereproduction time corresponding to a target packet to be transmitted isembedded in a subsequent packet to be transmitted after the targetpacket; and at the receiving end, when a transmission error of thetarget packet has occurred and the additional information of the targetpacket has an error, a retransmission request for the target packet asan error packet is made on the basis of the additional information ofthe target packet which is embedded in the subsequent packet, when thesubsequent packet transmitted after the target packet is received.Therefore, the transmission quality in a radio section in real-timetransmission is improved and, further, the number of retransmissiontimes is reduced.

According to a twelfth aspect of the present invention, there isprovided a data transmission apparatus for relaying data which aresuccessively transmitted from the transmitting end in units of packets,each packet having additional information relating to its sequencenumber, priority, data reproduction time at the receiving end. Thisapparatus comprises: a receiving unit for receiving the packetstransmitted from the transmitting end; a priority decision unit fordeciding the priority of each of the received packets; a reproductiontime decision unit for deciding packets which cannot be in time forreproduction at, the receiving end, amongst the packets to betransmitted to the receiving end; a retransmission packet storage unitfor storing packets the priorities of which are equal to or higher thana predetermined value, as retransmission, packets, on the basis of thepriority of each packet decided by the priority decision unit; aretransmission instruction receiving unit for receiving a retransmissionrequest from a terminal at the receiving end; a retransmission decisionunit for deciding whether retransmission of the packet for which theretransmission request has been made should be performed or not, on thebasis of the retransmission request and the storage status of theretransmission packets in the retransmission packet storage unit; atransmission queue management unit for setting the transmission order ofthe received packets and the packets which have been decided as packetsto be retransmitted, on the basis of the additional information; and atransmission unit for transmitting, in the transmission order set by themanagement unit, the data of packets other than the packets which aredecided as packets that cannot be in time for reproduction at thereceiving end, by the reproduction time decision unit. Therefore, thetransmission quality in a radio section in real-time transmission isimproved and, further, the number of retransmission times is reduced.

According to a thirteenth aspect of the present invention, there isprovided a data receiving apparatus for receiving data which aretransmitted from the transmitting end in units of packets, each packethaving additional information relating to its sequence number, priorityand data reproduction time at the receiving end, and successivelyreproducing the data for each packet. This apparatus comprises: areceiving unit for receiving the packets, transmitted from thetransmitting end; an error packet detection unit for detecting errorpackets in which errors have occurred during transmission, andoutputting normal packets which have been transmitted withouttransmission errors, on the basis of the data of the received packets; areproduction time decision unit for detecting the reproduction timegiven to each error packet detected by the error, packet detection unitand the arrival time of the error packet at the receiving end, andsetting the arrival time limit based on the reproduction time, anddeciding whether or not the error packet has arrived at the receivingend before the arrival time limit; and a retransmission instructionoutput unit for outputting a retransmission request only for the errorpacket which has arrived at the receiving end before the arrival timelimit, to the transmitting end, by indicating the sequence number of theerror packet, on the basis of the result of the decision in thereproduction time decision unit. Therefore, the transmission quality ina radio section in real-time transmission is improved and, further, thenumber of retransmission times is reduced.

According to a fourteenth aspect of the present invention, there isprovided a data transmission method for performing continuous datatransmission from the transmitting end to the receiving end in units ofpackets, each packet having additional information relating to itssequence number, priority, and data reproduction time at the receivingend, while successively reproducing data of packets arrived at thereceiving end. This method comprises: at the transmitting end, giving adata reproduction time and priority information to each packet to betransmitted; and storing, as retransmission data, only data of packetsthe priorities of which are equal to or higher than a predeterminedvalue, in a retransmission buffer; at the receiving end, when atransmission error is detected, detecting the priority information of anerror packet, the reproduction time of the error packet, and the arrivaltime of the error packet; setting the arrival time limit of the errorpacket on the basis of the reproduction time; and when the detectedpriority is equal to or higher than the predetermined value and theerror packet has arrived before the arrival time limit, outputting aretransmission request for this error packet to the transmitting end byindicating the sequence number of this error packet; at the transmittingend, when data of the packet having the sequence number indicated by theretransmission request from the receiving end is stored in theretransmission buffer, retransmitting only data of the packet thetransmission time of which does not pass the reproduction time, to thereceiving end, while discarding data of the packet the transmission timeof which has passed the reproduction time; and discarding the datastored in the retransmission buffer in order starting from a packetwhich cannot be in time for reproduction at the receiving end.Therefore, the transmission quality in a radio section in real-timetransmission is improved and, further, the number of retransmissiontimes is reduced.

According to a fifteenth aspect of the present invention, the datatransmission method of the fourteenth aspect, at the transmitting end,additional information relating to the sequence number, the priority,and the reproduction time of a predetermined packet is embedded in asubsequent packet to be transmitted after the predetermined packet; andat the receiving end, when a transmission error of the predeterminedpacket has occurred and the additional information of the predeterminedpacket has an error, a retransmission request for the predeterminedpacket as an error packet is made on the basis of the additionalinformation of the predetermined packet which is embedded in thesubsequent packet, when the subsequent packet transmitted after thepredetermined packet is received. Therefore, the transmission quality ina radio section in real-time transmission is improved and, further, thenumber of retransmission times is reduced.

According to a sixteenth aspect of the present invention, in the datatransmission method of the fifteenth aspect, at the transmitting end,the process of embedding the sequence number of a predetermined highpriority packet in a subsequent packet which follows the predeterminedhigh priority packet is continuously performed until a, high prioritypacket next to the predetermined high priority packet is transmitted;and at the receiving end, the sequence number of another packet which isembedded in the received packet is extracted, and when a transmissionerror has occurred in the packet of the extracted sequence number, aretransmission request for this packet is made by indicating thesequence number of this packet. Therefore, the transmission quality in aradio section in real-time transmission is improved and, further, thenumber of retransmission times is reduced.

According to a seventeenth aspect of the present invention in the datatransmission method of the fifteenth aspect, at the transmitting end,when additional information relating to the sequence number and thereproduction time corresponding to each packet is embedded in asubsequent packet to be transmitted after the packet; a differencebetween additional information relating to the sequence number and thereproduction time corresponding to the subsequent packet and theadditional information relating to the sequence number and thereproduction time corresponding to the previous packet is embedded asthe additional information. Therefore, the transmission quality in aradio section in real-time transmission is improved and, further, thenumber of retransmission times is reduced.

According to an eighteenth aspect of the present invention, there isprovided a data transmission method for performing data transmissionbetween a distribution server and a terminal through a relay server inunits of packets, and successively reproducing data of packets receivedat the terminal. This method comprising: when a transmission error hasoccurred between the relay server and the terminal, performingretransmission of an error packet by the relay server in response to aretransmission request from the terminal; and when a transmission errorhas occurred between the distribution server and the relay server,performing retransmission of an error packet by the distribution serverin response to a retransmission request which has been transmitted fromthe terminal through the relay server. Therefore, the number ofretransmission times is reduced between the distribution server and therelay server.

According to a nineteenth aspect of the present invention, in the datatransmission method of the eighteenth aspect, when a transmission errorhas occurred between the distribution server and the relay server, aretransmission request is transmitted from the relay server to thedistribution server, and retransmission of an error packet to the relayserver is performed by the distribution server. Therefore, wastefultransmission of error packets from the relay server to the terminal isavoided.

According to a twentieth aspect of the present invention, there isprovided a data transmission apparatus for relaying data which aresuccessively transmitted from a distribution server, in units of packetseach having additional information relating to its sequence number,priority, data reproduction time at the receiving end. This apparatuscomprises: a receiving unit for receiving the packets transmitted fromthe transmitting end; a priority decision unit for deciding the priorityof each of the received packets; a retransmission packet storage unitfor storing packets the priorities of which are equal to or higher thana predetermined value, as retransmission packets, on the basis of thepriority of each packet decided by the priority decision unit; aretransmission instruction receiving unit for receiving a retransmissionrequest from a terminal at the receiving end; a retransmission decisionunit for deciding whether retransmission of the packet for which theretransmission request has been made is to be performed or not, on thebasis of the retransmission request and the storage status of theretransmission packets in the retransmission packet storage unit; aretransmission instruction output unit for outputting the retransmissionrequest for the error packet requested by the terminal, to thedistribution server, on the basis of the result of the decision in theretransmission decision unit; a transmission queue management unit forsetting the transmission order of the received packets and the packetswhich have been decided as packets to be retransmitted, on the basis ofthe additional information; and a transmission unit for transmitting thedata of these packets in the transmission order set by the managementunit. Therefore, the number of retransmission times between thedistribution server and the relay server can be reduced.

According to a twenty-first aspect of the present invention, there isprovided a data transmission method for performing continuous datatransmission from the transmitting end to the receiving end in units ofpackets, each packet having additional information relating to itssequence number, priority, and data reproduction time at the receivingend, while successively reproducing data of packets received at thereceiving end. This method comprises: at the transmitting end, when apacket the priority of which is equal to or higher than a predeterminedvalue is transmitted as a high priority packet, storing data of thishigh priority packet, as retransmission data, in a retransmissionbuffer; managing the value of the transmitting end high prioritysequence number which corresponds to the number of transmitted highpriority packets, and the value of the sequence number of the highpriority packet so that these values are correlated with each other; andtransmitting a subsequent packet which follows the high priority packetafter embedding the value of the transmitting end high priority sequencenumber in this subsequent packet; at the receiving end, extracting thevalue of the transmitting end high priority sequence number which isembedded in the received packet; managing the value of the receiving endhigh priority sequence number which corresponds to the number ofreceived high priority packets; when the value of the extractedtransmitting end high priority sequence number is not equal to the valueof the receiving end high priority sequence number, outputting aretransmission request to the transmitting end, by indicating the valueof the transmitting end high priority sequence number which is obtainedon the basis of the value of the receiving end high priority sequencenumber; and updating the value of the receiving end high prioritysequence number, at the transmitting end, only when data of the packethaving the sequence number corresponding to the value of thetransmitting end high priority sequence number which is indicated by theretransmission request from the receiving end is stored in theretransmission buffer, retransmitting the data of this packet to thereceiving end. Therefore, retransmission of the high priority packet thepriority of which is equal to or higher than a predetermined value, canbe performed by simpler procedures.

According to a twenty-second aspect of the present invention, in thedata transmission method of the twenty-first aspect, at the receivingend, when the value of the transmitting end high priority sequencenumber embedded in the received packet is not equal to the value of thereceiving end high priority sequence number, a retransmission request isoutput to the transmitting end, by listing the values ranging from thevalue obtained by adding 1 to the receiving end high priority sequencenumber, to the value of the transmitting end high priority sequencenumber, as the values of the transmitting end high priority sequencenumbers, or by designating the range as the range of the values of thetransmitting end high priority sequence numbers; and at the transmittingend, the sequence numbers corresponding to the values of the pluraltransmitting end high priority sequence numbers which are indicated bythe retransmission request from the receiving end are retrieved, andonly when data of the packets having the sequence numbers obtained bythe retrieval are stored in the retransmission buffer, the data of thepackets are retransmitted to the receiving end. Therefore,retransmission of the high priority packet the priority of which isequal to or higher than a predetermined value, can be performed bysimpler procedures.

According to a twenty-third aspect of the present invention, in the datatransmission method of the twenty-first aspect, at the receiving end,the retransmission request is performed continuously several times,indicating the value of a transmitting end high priority sequencenumber; and at the transmitting end, the sequence number correspondingto the value of the transmitting end high priority sequence number whichis indicated by the retransmission request from the receiving end isretrieved, and data of the packet having the sequence number obtained bythe retrieval is retransmitted to the receiving end and, simultaneously,the correspondence between the value of the sequence number obtained bythe retrieval and the value of the transmitting end high prioritysequence number indicated by the receiving end is deleted. Therefore,when at least one of several transmission requests from the receivingend is normally received at the transmitting end, only the error packetthe priority of which is equal to or higher than a predetermined valuecan be retransmitted, whereby the transmission quality in a radiosection in real-time transmission can be effectively improved.

According to a twenty-fourth aspect of the present invention, there isprovided a data transmission apparatus for relaying data which aresuccessively transmitted from the transmitting end in units of packetseach packet having additional information relating to its sequencenumber, priority, data reproduction time at the receiving end. Thisapparatus comprises: a receiving unit for receiving the packetstransmitted from the transmitting end; a transmission queue managementunit for setting the transmission order of the received packets andpackets which are decided as packets to be retransmitted; a transmissionunit for transmitting data of these packets in the transmission orderset by the transmission queue management unit; a priority decision unitfor deciding the priority of each of the received packets; aretransmission packet storage unit for storing packets the priorities ofwhich are equal to or higher than a predetermined value, asretransmission packets, on the basis of the priority of each packetdecided by the priority decision unit; a sequence number management unitfor managing the value of the transmitting end high priority sequencenumber which corresponds to the number of transmitted high prioritypackets, and the value of the sequence number of the high prioritypacket so that these values are correlated with each other; a highpriority sequence number insertion unit for embedding the value of thetransmitting end high priority sequence number in a subsequent packetwhich follows thee high priority packet; a retransmission instructionreceiving unit for receiving a retransmission request indicating thehigh priority sequence number, from a terminal at the transmitting endand a retransmission decision unit for deciding whether retransmissionof the packet for which the retransmission request has been made is tobe performed or not, on the basis of the retransmission request and thestorage status of the retransmission packets in the retransmissionpacket storage unit. Therefore, only the error packet the priority ofwhich is equal to or higher than a predetermined value can beretransmitted, whereby retransmission of the high priority packet can beperformed by simpler procedures.

According to a twenty-fifth aspect of the present invention, there isprovided a data receiving apparatus for receiving data which aretransmitted from the transmitting end in units of, packets, each packethaving additional information relating to its sequence number, priority,and data reproduction time at the receiving end, and successivelyreproducing the data for each packet. This apparatus comprises: areceiving unit for receiving the packets transmitted from thetransmitting end; an error packet detection unit for detecting an errorpacket in which an error has occurred during transmission, andoutputting a normal packet which has been transmitted withouttransmission errors and the value of the transmitting end high prioritysequence number which corresponds to the number of transmitted highpriority packets and is embedded in the normal packet; a high prioritysequence number management unit for managing the value of the receivingend high priority sequence number which corresponds to the number ofnormal high priority packets which have been received withouttransmission errors, on the basis of the output from the error packetdetection unit; a retransmission sequence number decision unit forcomparing the value of the transmitting end high priority sequencenumber from the error packet detection unit with the value of thereceiving end high priority sequence number, and when these values arenot equal, deciding the value of the transmitting end high prioritysequence number for which a retransmission request is to be made, on thebasis of the value of the receiving end high priority sequence number;and a retransmission instruction output unit for outputting aretransmission request to the transmitting end, by indicating the valueof the decided transmitting end high priority sequence number.Therefore, retransmission of the high priority packet can be performedwith simpler procedures.

According to a twenty-sixth aspect of the present invention, there isprovided a data structure of a packet for performing data transmissionfrom the transmitting end and the receiving end, wherein the packetcomprises a header section containing relevant information indicatingthe attribute of the packet, and a data section containing data to betransmitted; and the header section comprises at least first and secondheader information, amongst first header information indicating thesequence number corresponding to the packet, second header informationindicating the priority of the packet, and third header informationindicating the reproduction time at the receiving end, of the data to betransmitted. Therefore, retransmission of a low priority packet: andretransmission of a packet which cannot be in time for reproduction canbe avoided, whereby the transmission quality in a radio section inreal-time transmission is improved while reducing the number ofretransmission times.

According to a twenty-seventh aspect of the present invention, in thepacket data structure of the twenty-sixth aspect, the header section ofthe packet includes attribute information of a packet which has alreadybeen transmitted before the packet. Therefore, retransmission of anerror packet can be performed with reliability.

According to a twenty-eighth aspect of the present invention, in thepacket data structure of the twenty-seventh aspect, the header sectionof the packet includes the first and second information or the first andthird information, as attribute information of a packet which hasalready been transmitted before the packet. Therefore, retransmissioncontrol based on the priority or the reproduction time can be performedwith reliability.

According to a twenty-ninth aspect of the present invention, in thepacket data structure of the twenty-sixth aspect, the header section ofthe packet includes the value of the high priority sequence; numbercorresponding to the number of high priority packets which have beentransmitted before the packet and having the priorities equal to orhigher than a predetermined value. Therefore, the transmission qualityin a radio section in real-time transmission is improved and, moreover,retransmission of an error packet is realized by simpler procedures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) are block diagrams for explaining data transmissionapparatuses as a relay server and a distribution server, respectively,in a data transmission system according to a first embodiment of thepresent invention.

FIG. 2 is a block diagram illustrating a data receiving apparatus(receiving terminal) in the data transmission system according to thefirst embodiment.

FIG. 3 is a sequence diagram for explaining packet selectiveretransmission control in a data transmission method according to thefirst embodiment.

FIG. 4 is a block diagram illustrating a data transmission apparatus(relay server) in a data transmission system according to a secondembodiment of the present invention.

FIG. 5 is a block diagram illustrating a data receiving apparatus(receiving terminal) in the data transmission system according to thesecond embodiment.

FIG. 6 is a sequence diagram for explaining packet selectiveretransmission control in a data transmission method according to athird embodiment of the present invention.

FIG. 7 is a block diagram illustrating a data transmission apparatus(relay server) in a data transmission system according to a modificationof the third embodiment.

FIG. 8 is a block diagram illustrating a data receiving apparatus(receiving terminal) in the data transmission system according to themodification of the third embodiment.

FIG. 9 is a sequence diagram for explaining packet selectiveretransmission control in a data transmission method according to themodification of the third embodiment.

FIG. 10 is a block diagram illustrating a data transmission apparatus(relay server) in a data transmission system according to a fourthembodiment of the present invention.

FIG. 11 is a block diagram illustrating a data receiving apparatus(receiving terminal) in the data transmission system according to thefourth embodiment.

FIG. 12 is a sequence diagram for explaining a first example of packetselective retransmission control in a data transmission method accordingto the fourth embodiment.

FIG. 13 is a sequence diagram for explaining a second example of packetselective retransmission control in the data transmission methodaccording to the fourth embodiment.

FIG. 14 is a sequence diagram for explaining a third example of packetselective retransmission control in the data transmission methodaccording to the fourth embodiment.

FIG. 15 is a sequence diagram for explaining a fourth example of packetselective retransmission control in a data transmission method accordingto a modification of the fourth embodiment.

FIGS. 16(a) and 16(b) are block diagrams for explaining a datatransmission apparatus as a relay server and a data receiving apparatusas a receiving terminal, respectively, in a data transmission systemaccording to a fifth embodiment of the present invention.

FIG. 17 is a sequence diagram for explaining packet selectiveretransmission control in a data transmission method according to thefifth embodiment.

FIG. 18 is a block diagram illustrating a data transmission apparatus(relay server) in a data transmission system according to a sixthembodiment of the present invention.

FIG. 19 is a block diagram illustrating a data receiving apparatus(receiving terminal) in the data transmission system according to thesixth embodiment.

FIG. 20 is a sequence diagram for explaining packet retransmissioncontrol with time limit, in the data transmission method according tothe sixth embodiment.

FIG. 21 is a sequence diagram for explaining packet retransmissioncontrol with time limit, in a data transmission method according to anmodification of the sixth embodiment.

FIG. 22 is a sequence diagram for explaining packet retransmissioncontrol with time limit, in a data transmission method according toanother modification of the sixth embodiment.

FIG. 23 is a block diagram illustrating a data transmission apparatus(relay server) in a data transmission system according to a seventhembodiment of the present invention.

FIG. 24 is a block diagram illustrating a data transmission apparatus(relay server) in a data transmission system according to a modificationof the seventh embodiment.

FIG. 25 is a block diagram illustrating a data transmission apparatus(relay server) in a data transmission system according to an eighthembodiment of the present invention.

FIG. 26 is a block diagram illustrating a data receiving apparatus(receiving terminal) in the data transmission system according to theeighth embodiment.

FIGS. 27(a) and 27(b) are diagrams for explaining a reproduction timeestimation method according to the eighth embodiment illustrating therelationship between the time stamp of a packet and the packet outputtime (FIG. 27(a)) and a time stamp mapping method (FIG. 27(b)).

FIGS. 28(a) and 28(b) are schematic diagrams for conceptually explainingthe conventional video transmission system, illustrating a server and aterminal connected by a cable (FIG. 28(a)), and a server and a terminalconnected by a network including a radio transmission section.

FIGS. 29(a) and 29(b) are block diagrams for explaining a datatransmission apparatus as a relay server and a data transmissionapparatus as a distribution server, respectively, in a data transmissionsystem according to a ninth embodiment of the present invention.

FIG. 30 is a block diagram illustrating a data receiving apparatus(receiving terminal) in the data transmission system according to theninth embodiment.

FIG. 31 is a sequence diagram for explaining packet selectiveretransmission control in the real-time data transmission methodaccording to the ninth embodiment.

FIG. 32 is a block diagram illustrating a data transmission apparatus(relay server) in a data transmission system according to a tenthembodiment of the present invention.

FIG. 33 is a block diagram illustrating a data receiving apparatus(receiving terminal) in the data transmission system according to thetenth embodiment.

FIGS. 34(a)-34(c) are diagrams illustrating the data structure of apacket for transmitting data by a data transmission method according toany of the aforementioned embodiments (FIG. 34(a)), the structure of thepacket header (FIG. 34(b)), and the convention of header information(FIG. 34(c)).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the inventor's viewpoint and the fundamental principle ofthe present invention will be described.

The inventors of the present invention have earnestly studied about amethod for improving the transmission quality in a network including aradio transmission section, and finally discovered that the transmissionquality in the radio section can be improved by performing real-timeretransmission of packets, in the existing real-time transmissionmethod.

To be specific, in the existing real-time transmission method, packetdata are transmitted in real time between a distribution server and aterminal unit through a relay server or the like, and additionalinformation (e.g., a sequence number, a time stamp, etc) required forreal-time transmission of each packet data is given to the header of thepacket. The additional information enables real-time control of packetretransmission.

In the data transmission method of the present invention in which datatransmission is performed in real time between the distribution serverand the terminal through the relay server while retransmission controlis performed in real time, the relay server may serve as the videodistribution server. Further, as for the video data transmissiondirection, the present invention can be applied to not only the forwardtransmission from the distribution server to the terminal but also thebackward transmission from the terminal to the distribution server.

Further, in the present invention, for effective real-time transmission,control of transmission QoS (Quality of Service) in the relay server isrealized.

Next, the summary of real-time retransmission control in the datatransmission method of the present invention will be briefly described.

While the real-time retransmission control of the present inventionregards the transmission quality of the radio section and recovers errorpackets due to radio transmission error by retransmission of thepackets, the conventional retransmission control is directed to realizehigh reliability in data transmission (error-free transmission). Thatis, in order to recover all error packets, retransmission of errorpackets is repeated until data are correctly transmitted to thereceiving end. In the conventional retransmission control when the finalerror packet retransmission has failed, the data transmission isabnormally ended as a communication error.

By the way, in the retransmission control required for the real-timevideo data transmission, even when a transmission error occurs whilepredetermined packets are transmitted and thereby, several frames ofimages are lost, this is not a fatal transmission error which leads toabnormal end of data transmission. Accordingly, in the video datatransmission method, to complete real-time transmission with a measureof reliability is given the highest priority.

For example, in transmission of a video signal based on MPEG standard,in the case where a packet corresponding to an I frame (intra-framecoded image) as a reference image becomes an error packet, even whensubsequent packets corresponding to a P frame (inter-frameforward-prediction coded image) and a B frame (inter-framebidirectional-prediction coded image) are received normally, videosignals of the P frame and the B frame cannot be reproduced. So, as forthe I frame, it is necessary to recover the transmission error to theutmost.

Further, in video transmission (stream type communication having anaudio signal and a video signal as objects to be transmitted), incontrast with the conventional retransmission control, it is necessaryto perform retransmission while performing real-time transmission.

In order to realize such real-time transmission, the present inventionprovides a real-time transmission method in which the followingretransmission control for error packets is performed.

First retransmission control is selective retransmission control forreducing the number of retransmission times by selecting, as packets tobe retransmitted, high priority packets amongst the error packets.Second retransmission control is retransmission control with time limitfor reducing excessive retransmission by stopping retransmission ofpackets which cannot be in time for reproduction, amongst the errorpackets.

Hereinafter, embodiments of the present invention will be described.

[Embodiment 1]

FIGS. 1-3 are diagrams for explaining a data transmission methodaccording to a first embodiment of the present invention.

In the data transmission method of this first embodiment, datatransmission from the transmitting end to the receiving end iscontinuously performed in units of packets, each packet havingadditional information relating to its sequence number, priority, anddata reproduction time at the receiving end, while successivelyreproducing data of packets received at the receiving end. At this time,only error packets the priorities of which are equal to or higher than apredetermined value are retransmitted.

FIG. 1(a) is a block diagram illustrating a data transmission apparatus101 in a data transmission system which performs real-time datatransmission according to the data transmission method of the firstembodiment.

This data transmission apparatus 101 constitutes a relay server(transmitting end) which relays data transmitted between a distributionserver and a terminal (receiving end). This apparatus 101 includes areceiving unit 11, a transmission queue management unit 12, and atransmission unit 13. The receiving unit 11 receives packets transmittedfrom the distribution server. The transmission queue management unit 12sets the transmission order of the received packets and the packets tobe retransmitted (hereinafter referred to as retransmission packets) inaccordance with the above-described additional information. Thetransmission unit 13 transmits the packet data in the transmission orderwhich has been set by the transmission queue management unit 12.

Each packet transmitted from the transmitting end is composed of a datasection containing digital data such as video data, audio data, and textdata, and a header section containing additional information other thanthese digital data. To be specific, the header section of each packetcontains additional information relating to its sequence number,priority, and data reproduction time at the receiving end.

Further, the data transmission apparatus 101 includes a buffer 17 forretransmission (hereinafter, referred to as a retransmission buffer 17),a packet priority decision unit 15, and a retransmission buffermanagement unit 18. The retransmission buffer 17 stores predeterminedpackets amongst the received packets, as retransmission packets. Thepacket priority decision unit 15 decides the priorities of the receivedpackets. The retransmission buffer management unit 18 controls theretransmission buffer 17 such that data of packets the priorities ofwhich are equal to or higher than a predetermined value are stored inthe buffer 17, in accordance with the decided priorities of the packets.To be specific, only when data of a packet having a sequence numberindicated by a retransmission request from the receiving side is storedin the retransmission buffer 17, the retransmission decision unit 16decides that data of this packet should be retransmitted to thereceiving end.

Further, the data transmission apparatus 101 includes a retransmissioninstruction receiving unit 14 and a retransmission decision unit 16. Theretransmission instruction receiving unit 14 receives a retransmissioninstruction (hereinafter also referred to as a retransmission request)from the terminal at the receiving end. The retransmission decision unit16 decides whether retransmission of a packet for which theretransmission instruction has been made is performed or not.

While in FIG. 1(a) the data transmission apparatus 101 constitutes arelay server, when the data transmission apparatus is a distributionserver, it is constructed as shown in FIG. 1(b). That is, in FIG. 1(b),the receiving unit 11 of the data transmission apparatus 101 is replacedwith a coded packet generation unit, 10 a which encodes the data andoutputs the coded data in packet units, and a priority allocation unit10 b which allocates additional information such as a priority to eachpacket output from the coded packet generation unit 10 a.

FIG. 2 is a block diagram illustrating a data receiving apparatus 201 inthe data transmission system which performs real-time data transmissionaccording to the data transmission method of the first embodiment.

The data receiving apparatus 201 includes a receiving unit 21, an errorpacket detection unit 22, and a packet decoding unit 23. The receivingunit 21 receives the packets transmitted from the relay server (the datatransmission apparatus at the transmitting end). The error packetdetection unit 22 detects error packets in which errors have occurredduring transmission, and outputs normal packets which have beentransmitted without transmission errors. The packet decoding unit 23receives the normal packets and decodes the coded data of the normalpackets.

Further, the data receiving apparatus 201 includes a reception historymanagement unit 24, a packet priority decision unit 25, and aretransmission instruction output unit 26. The reception historymanagement unit 24 manages the packet reception history. The packetpriority decision unit 25 receives the result of the detection in theerror packet detection unit 22 and decides an error packet the priorityof which is equal to or higher than a predetermined value. Theretransmission instruction output unit 26 outputs a request forretransmitting the error packet which has been decided in the packetpriority decision unit 25, toward the transmitting end, by indicatingthe sequence number of the error packet.

Next, the function, and effect will be described.

FIG. 3 is a sequence chart for explaining the packet selectiveretransmission control in the data transmission method of the firstembodiment.

In the data transmission method of this first embodiment, when atransmission error occurs during packet transmission, a retransmissionrequest is made for only the packets the priorities of which are equalto or higher than a predetermined value, from the receiving end to thetransmitting end, while no retransmission request is made for the errorpackets the priorities of which are lower than the predetermined value.

For example, assuming that the priorities equal to or higher than thepredetermined value are high priorities while the priorities lower thanthe predetermined value are low priorities, when an error occurs duringtransmission of a high priority packet (S1) of sequence number, S1, aretransmission request for this high priority packet (S1) is made.However, when an error occurs during transmission of a low prioritypacket (S2) of sequence number S2, no retransmission request is made forthis low priority packet (S2).

To be specific, each packet transmitted from the distribution server isgiven additional information relating to its sequence number andpriority. In the data transmission apparatus 101 as a relay server, thetransmission order of the received packets is set by the transmissionqueue management unit 12, and the packets are supplied to thetransmission unit 13. On the other hand, the priorities of the receivedpackets are decided by the packet priority decision unit 15. Then, inthe transmission unit 13, transmission of these packets is performedaccording to the transmission order, which has been set. Further, thosepackets the priorities of which are decided as being equal to or higherthan the; predetermined value are stored in the retransmission buffer 17under control of the retransmission buffer management unit 17. Further,in the retransmission buffer 17, data are successively released(discarded) from the packets which cannot be in time for reproduction,under control of the management unit 18.

When the retransmission buffer 17 has no more vacant space (capacity),one of the following two processes is performed: a first updatingprocess in which the above-described retransmission data is retainedwhile the data of the packets stored in the retransmission buffer 17 arediscarded in order starting from a packet of the earliest reproductiontime; and a second updating process in which the retransmission data isretained while successively discarding the data stored in theretransmission buffer 17 so that reproduction of packet data forpredetermined packets, amongst the packet of the earliest reproductiontime and the subsequent packets, is performed at regular intervals atthe receiving end.

In this way, the packets from the distribution server are successivelytransmitted to the terminal (data receiving apparatus) 201 through therelay server (data transmission apparatus) 101.

In the data receiving apparatus 201, the packets from the relay server(data transmission apparatus) 101 are received by the receiving unit 21,and the received packets are supplied to the error packet detection unit22. Then, only the packets, which have been transmitted withouttransmission errors are output from the error packet detection unit 22to the packet decoding unit 23, and the additional information of eachpacket is supplied to the reception history management unit 24. At thistime, the priority information of each error packet is supplied to thepacket priority decision unit 25, wherein it is decided whether or notthe priority of the error packet is equal to or higher than apredetermined value. With respect to the error packet the priority ofwhich is equal to or higher than the predetermined value, theretransmission instruction output unit 26 outputs a retransmissionrequest to the transmitting end, by indicating the sequence number ofthis error packet.

Then, in the data transmission apparatus 101 at the transmitting end,the retransmission request is received by the retransmission instructionreceiving unit 14, and it is decided by the retransmission decision unit16 as to whether the packet of the sequence number indicated by theretransmission request is stored in the retransmission buffer 17 or not.When the packet of the sequence number indicated by the retransmissionrequest is stored in the retransmission buffer 17, this packet is outputas a retransmission packet, from the retransmission buffer 17 to thetransmission queue management unit 12. In the transmission queuemanagement unit 12, the retransmission packet is given a predeterminedtransmission order and then retransmitted to the receiving end throughthe transmission unit 13.

As described above, according to the first embodiment of the presentinvention, data transmission from the transmitting end to the receivingend is continuously performed in units of packets each having additionalinformation relating to its sequence number, priority, and datareproduction time and, simultaneously, data of the packets received atthe receiving end are successively reproduced. With respect to errorpackets affected by transmission errors, only those having prioritiesequal to or higher than a predetermined value are retransmitted.Therefore, the transmission quality of the radio section in thereal-time transmission is improved and, moreover, the number ofretransmission times can be reduced.

In this first embodiment, each packet may be given the frame type, suchas I frame, P frame, and B frame, as the additional information, insteadof the priority.

Further, there are various methods for deciding the packet priority. Forexample, in the case of a video signal based on the MPEG standard,packets corresponding to I frames may be decided as high prioritypackets.

Further, in the packet discarding process performed when theretransmission buffer is filled to the capacity, the above-describedfirst or second updating process may be performed on the packets in theorder of ascending priorities.

[Embodiment 2]

FIGS. 4-5 are diagrams for explaining a data transmission methodaccording to a second embodiment of the present invention.

FIG. 4 is a block diagram illustrating a data transmission apparatus 102in a data transmission system which performs real-time data transmissionaccording to this data transmission method.

The data transmission apparatus 102 includes an error correction unit31, in addition to the constituents of the data transmission apparatus101 of the first embodiment. The error correction unit 31 performs anerror correction process in which each packet output from thetransmission queue management unit 12 is given error correction codesfor additional information such as its sequence number priority, etc.,and the packet which has been subjected to the error correction processis supplied to the transmission unit 13. Other constituents of the datatransmission apparatus 102 are identical to those of the datatransmission apparatus 101 of the first embodiment.

FIG. 5 is a block diagram illustrating a data receiving apparatus 202 inthe data transmission system which performs the real-time datatransmission according to the data transmission method of this secondembodiment.

The data receiving apparatus 202 of this second embodiment includes anerror correction unit 41, in addition to the constituents of the datareceiving apparatus 201 of the first embodiment. The error correctionunit 41 performs an error correction process in which each packetreceived by the receiving unit 21 is subjected to an error correctionprocess in which the additional information of this packet is subjectedto error correction by using the error correction codes given to thepacket, and the packet which has been subjected to the error correctionprocess is output to the error packet detection unit 22. Otherconstituents of the data receiving apparatus 202 are identical to thoseof the data receiving apparatus 201 of the first embodiment.

Next, the function and effect will be described.

In the data transmission method according to the second embodiment, atthe transmitting end, each packet to be transmitted is given errorcorrection codes for the additional information relating to its sequencenumber, priority, etc. At the receiving end, the additional informationis subjected to error correction according to the error correction codesand, thereafter, a retransmission request for the error packet is madein accordance with the additional information. Thereby, even when thesequence number and the priority information have errors, aretransmission request for the error packet can be correctly performed.

[Embodiment 3]

FIGS. 6-9 are diagrams for explaining data transmission methodsaccording to a third embodiment of the invention and a modification ofthe, third embodiment. FIG. 6 is a sequence diagram for explainingpacket selective retransmission control in the data transmission methodof the third embodiment.

In the data transmission method of the third embodiment, at thetransmitting end, additional information relating to the sequence numberand priority of a predetermined packet is embedded in a subsequentpacket which will be transmitted after the predetermined packet. At thereceiving end, when a transmission error of the predetermined packetoccurs and thereby the additional information of the predeterminedpacket has an error, a retransmission request for the error packet ismade when receiving the subsequent packet which is transmitted after theerror packet, in accordance with the additional information of thepredetermined packet which is embedded in the subsequent packet.

For example, assuming that the priorities equal to or higher than apredetermined value are high priorities while the priorities lower thanthe predetermined value are low priorities, as shown in FIG. 6, when anerror occurs during transmission of a high priority packet (S1) ofsequence number S1 and only a low priority packet (S2) of sequencenumber S2 which follows the packet S1 is normally received, aretransmission request for the high priority packet (S1), is made whenthe next low priority packet (S2) is received.

On the other hand, when an error occurs during transmission of a lowpriority packet (S3) of sequence number S3 and only a high prioritypacket (S4) of sequence number S4 which follows the packet S3 isnormally received, no retransmission request for the low priority packetS3 is made when the next high priority packet (S4) is received.

In the data transmission method so constructed, even when an erroroccurs during transmission of a predetermined packet and thereby thesequence number or the priority information of this packet has an error,since the additional information (sequence number, priority, etc.) ofthe predetermined packet is embedded in the subsequent packet which istransmitted next to this packet, a transmission request for this errorpacket (predetermined packet) can be made correctly.

While in this third embodiment the additional information of each packetis embedded in the next packet, it may be embedded in, not only the nextpacket, but, also a plurality of subsequent packets. This constructionwill be described hereinafter as a modification of the third embodiment.

[Modification of Embodiment 3]

FIG. 7 is a block diagram illustrating a data transmission apparatus 103in a data transmission system which performs real-time data transmissionby using a data transmission method according to the modification of thethird embodiment.

The data transmission apparatus 103 includes a sequence number storageunit 33 and a sequence number insertion unit 32, in addition to theconstituents of the data transmission apparatus 101 of the firstembodiment. The sequence number storage unit 33 stores the sequencenumbers of the packets the priorities of which are equal to or higherthan a predetermined value, amongst the packets transmitted by atransmission unit 13 a. The sequence number insertion unit 32 outputseach of the sequence numbers stored in the sequence number storage unit33 to the transmission unit 13 a so that the sequence number is insertedin the header of the packet to be transmitted. Further, the transmissionunit 13 a of this modification is different from the transmission, unit13 of the first embodiment only in that it inserts the sequence numbersupplied from the sequence number insertion unit 32 into the header ofthe packet supplied from the transmission queue management unit 12,before transmitting the packet. Other constituents of the datatransmission apparatus 103 are identical to those of the, datatransmission apparatus 101 of the first embodiment.

FIG. 8 is a block diagram illustrating a data receiving apparatus 203 inthe data transmission system which performs the real-time datatransmission by using the data transmission method according to themodification of the third embodiment.

The data receiving apparatus 203 according to the modification of thethird embodiment includes an inserted sequence extraction unit 42, inaddition to the constituents of the data receiving apparatus 201 of thefirst embodiment. The inserted sequence, extraction unit 42 extracts,from a normal packet output from the error packet detection unit 22, thesequence number of a high priority packet which has been received inadvance of the normal packet. The normal packet is output to the packetdecoding unit 23 through the inserted sequence extraction unit 42.Further, in the data receiving apparatus 203, the packet prioritydecision unit 25 a outputs a retransmission request to theretransmission instruction output unit 26 when the packet of thesequence number extracted by the inserted sequence extraction unit 42 isan error packet. Other constituents of the data receiving unit 203 areidentical to those of the data receiving apparatus 201 of the firstembodiment.

Next, the function and effect will be described.

FIG. 9 is a sequence diagram for explaining packet selectiveretransmission control in the data transmission method according to themodification of the third embodiment.

In the data transmission apparatus (transmitting end) 103, in additionto the transmission operation of the data transmission apparatus 101according to the first embodiment, the process of embedding the sequencenumber of a high priority packet to be transmitted in the subsequentpackets, is carried out until the next high priority is transmitted.

For example, assuming that the priorities equal to or higher than apredetermined value are high priorities while the priorities lower thanthe predetermined value are low priorities, as shown in FIG. 9, after ahigh priority packet (S1) of sequence number S1, has been transmitted;subsequent packets (S2)˜(S4) of sequence numbers S2˜S4 are transmittedafter the sequence number S1 of the previous high priority packet (S1)is embedded therein, and then a high priority packet (S5) of sequencenumber S5 is transmitted after the sequence number S4 of the previoushigh priority packet (S4) is embedded therein.

When an error occurs while the high priority packet (S1) and thesubsequent low priority packets (S2) and (S3) are transmitted and soonly the high priority packet (S4) is normally received, aretransmission request for the first high priority packet S1 is madewhen the next high priority packet (S4) is received.

Further, in the data receiving apparatus (receiving end) 203, inaddition to the receiving operation of the data receiving apparatus 201of the first embodiment, the following retransmission is performed. Thatis, when the sequence number embedded in the received packet isextracted, if the packet corresponding to this sequence number is anerror packet, a packet retransmission request is sent to thetransmitting end by using the sequence number of this packet.

In the data transmission method according to the modification of thethird embodiment, at the transmitting end, the process of embedding thesequence number of a high priority packet, the priority of which isequal to or higher than a predetermined value, into the subsequentpackets which follow this high priority packet, is continued until ahigh priority packet next to the high priority packet is transmitted. Atthe receiving end, the, sequence number of another packet (high prioritypacket) embedded in the received packet is extracted. When this packet(another packet) is an error packet, a retransmission request for thispacket is made by indicating the sequence number of: this packet.Therefore, even when two successive packets become error packets, thesequence number of the high priority packet which has become an errorpacket can be detected from the header information of the subsequentpacket which is transmitted without a transmission error, whereby aretransmission request for the high priority error packet can be madewith higher reliability.

In the third embodiment, the sequence number and priority information ofeach packet are embedded in the header of a packet to be transmittednext to this packet, and in the modification of the third embodiment,the sequence number of a high priority packet to be transmitted isembedded in the subsequent plural packets until the next high prioritypacket is transmitted. However, the information to be embedded in thesubsequent packet is not restricted thereto. For example, the number ofretransmission times may be embedded in the subsequent packet togetherwith the sequence number.

In this case, at the transmitting end, information relating to thenumber of retransmission times is embedded in each packet together withthe sequence number. At the receiving end, when making a retransmissionrequest for an error packet, the number of retransmission times and thesequence number corresponding to the error packet are paired andindicated to the transmitting end. At the transmitting end, a selectiveretransmission process is performed as follows. That is, retransmissionof the error packet for which the retransmission request has been madeis performed when the data of the packet having the sequence numberindicated by the retransmission request from the receiving end is storedin the retransmission buffer and there is matching in the number ofretransmission times between the packet for which the retransmissionrequest is made and the packet which is stored in the buffer and has theidentical sequence number. Then, the number of retransmission times ofthe retransmitted packet in the retransmission buffer is incremented.

Hereinafter, a specific construction for embedding the number ofretransmission times (hereinafter also referred to as retransmissioncount) in the subsequent packets will be described as a fourthembodiment of the invention.

[Embodiment 4]

FIGS. 10-15 are diagrams for explaining a data transmission methodaccording to a fourth embodiment of the present invention.

FIG. 10 is a block diagram illustrating a data transmission apparatus104 in a data transmission system which performs real-time datatransmission by the data transmission method according to the fourthembodiment.

The data transmission apparatus 104 includes a sequencenumber/retransmission count storage unit 35, a sequencenumber/retransmission count insertion unit 34, and a retransmissioncount clear unit 38. The sequence number/retransmission count storageunit 35 stores the sequence numbers and the retransmission counts, ofthe packets the priorities of which are equal to or higher than apredetermined value, amongst the packets transmitted by the transmissionunit 13 a. The sequence number/retransmission count insertion unit 34outputs the sequence number and the retransmission count which arestored in the storage unit 35, to the transmission unit 13 a, such thatthese data are inserted in the header of the packet to be transmitted.The retransmission count clear unit 38 subjects the packet received atthe receiving unit 11 to a process of clearing the retransmission count,and outputs the packet to the transmission queue management unit 12.Further, the transmission unit 13 a of this fourth embodiment isdifferent in function from the transmission unit 13 of the firstembodiment only in that it inserts the sequence number and theretransmission count supplied from the sequence number/retransmissioncount insertion unit 34 into the header of the packet supplied from thetransmission queue management unit 12 and then transmits this packet.

The data transmission apparatus 104 further includes a sequencenumber/retransmission count comparison unit 36 which receives theretransmission-instruction received by the retransmission instructionreceiving unit 14 (i.e., the sequence number and retransmission count ofthe packet to be retransmitted) through the reproduction decision unit16 a, and compares the sequence number of the packet for which theretransmission request, has been made with the sequence numbers of thepackets stored in the retransmission buffer 17. Further, it compares theretransmission count of the packet for which the retransmission requesthas been made with the retransmission count of the packet which isstored in the retransmission buffer 17 and has the sequence numberindicated by the retransmission request.

In the data transmission apparatus 104, the retransmission decision unit16 a decides retransmission of packets as follows, in accordance withthe output from the comparison unit 36. To be specific, when thesequence number of the rearmost packet which has been received mostrecently is larger than the sequence number of the error packet, it isdecided that selective retransmission should be performed on the errorpacket. On the other hand, when the sequence number of the rearmostpacket is smaller than the sequence number of the error packet, it isdecided that selective retransmission should be performed on a packethaving a sequence number which is larger than the sequence number of therearmost packet and equal to or smaller than the sequence number of theerror packet. Further, the retransmission buffer 17 a outputs the packetto be retransmitted, to the transmission queue management unit 12, inaccordance with the result of the decision in the retransmissionpropriety decision unit 16 a.

In FIG. 10, the sequence number/retransmission count increment unitincrements the retransmission count (number of retransmission times) ofthe packet in the retransmission buffer 17 a, which packet is decided tobe retransmitted by the retransmission decision unit 16 a.

Other constituents of the data transmission apparatus 104 are identicalto those of the data transmission apparatus of the first embodiment.

FIG. 11 is a block diagram illustrating a data receiving apparatus 204in the data transmission system which performs the real-time datatransmission according to the data transmission method of the fourthembodiment.

The data receiving unit 204 of this fourth embodiment includes aretransmission instruction output unit 26 a, instead of theretransmission instruction output unit 26 of the data receivingapparatus 203 according to the modification of the third embodiment. Theretransmission instruction output unit 206 a outputs a retransmissioninstruction which requests retransmission of a packet, by usinginformation in which the sequence number and retransmission count ofthis packet are paired.

Other constituents of the data receiving apparatus 204 are identical tothose of the data receiving apparatus 203 according to the modificationof the third embodiment.

Next, the function and effect will be described.

FIG. 12 a sequence diagram for explaining packet selectiveretransmission control in the data transmission method of the fourthembodiment, illustrating a first example of data exchange between thetransmitting end and the receiving end.

In the state shown in FIG. 12, the high priority packet of sequencenumber S1 has already been received normally at the receiving end.Hereinafter, a packet having a sequence number Sn and retransmissioncount N is represented as a packet (Sn,N) [n,N: natural numbers]. In thefigure, packets of sequence numbers S1, S2, and S4 are high prioritypackets, and packets of sequence numbers, S3 and S5 are packets otherthan the high priority packets. In this fourth embodiment, only theadditional information of the above-mentioned high priority packets areembedded in the subsequent packets.

Initially, as shown in FIG. 12, at a predetermined transmission timing,the high priority packet (S2,1) subsequent to the high priority packet(S1) is retransmitted. At this time, in the header of the packet (S2,1),the sequence number S1 and retransmission count of the high prioritypacket (S1) are embedded as well as its sequence number S2 andretransmission count 1.

At the next transmission timing, the packet (S3) subsequent to the highpriority packet (S2,1) is transmitted having, in its header, thesequence number S2 and retransmission count 1 of the high prioritypacket (S2,1) as well as its sequence number S3.

At the next transmission timing, the next high priority packet (S4,1) isretransmitted, having, in its header, the sequence number S2 andretransmission count 1 of the high priority packet (S2,1) as well as itssequence number S4 and retransmission count 1.

At the next transmission timing, the next packet (S5) is transmitted,having, in its header, the sequence number S4 and retransmission count 1of the high priority packet (S4,1) as well as its sequence number S5.

In the above-described four times of packet transmission, transmissionerror has occurred during the first to third packet transmission and,therefore, the packets (S2,1), (S3), and (S4) are not received at thereceiving end while only the packet (S5) is received.

In this state, at the transmitting end, it is known that the highpriority packet which has been received most recently is the packethaving the sequence number S1, and the error packet is the packet havingthe sequence number S4 and the retransmission count 1, but it is notknown what kinds of packets have been transmitted between the highpriority packet (S1) and the packet (S5).

So, the receiving end sends a retransmission request for the highpriority packet (S4,1), together with the sequence number S1 of themost-recently received high priority packet as well as the sequencenumber S4 and retransmission count 1 of this packet (S4,1).

Then, at the transmitting end, the sequence number S1 of the mostrecently received high priority packet is compared with the sequencenumber S4 of the error packet for which the retransmission request hasbeen made. In this case, since the sequence number of the most-recentlyreceived high priority packet is smaller than the sequence number of theerror packet, the transmitting end performs selective retransmission forthose packets having sequence numbers larger than the sequence number S1and equal to or smaller than the sequence number S4.

In this case, the high priority packet (S2,2) is transmitted having, inits header, the sequence number S4 and retransmission count 1 of thehigh priority packet (S4,1) and, subsequently, the high priority packet(S4,2) is transmitted, having, in its header the sequence number S2 andretransmission count 2 of the high priority packet (S2,2).

On receipt of the high priority packet (S2,2), the receiving end sends aretransmission request for the high priority packet (S4,1) toward thetransmitting end, together with the sequence number S2 of themost-recently received high priority packet (S2,2) as well as thesequence number S4 and retransmission count 1 of this packet (S4,1).However, with respect to the high priority packet (S4), since the secondretransmission has already been done, no retransmission is performed inresponse to the retransmission request for the high priority packet(S4,1).

Next, another example of data exchange will be described by using asequence diagram of FIG. 13.

In the case shown in FIG. 13, data exchange from transmission of thehigh priority packet (S2,1) to transmission of the high priority packet(S2,2) is identical to that describe with respect to FIG. 12.

In the case shown in FIG. 13, transmission of the high priority packet(S2,2) is error transmission, and the next high priority packet (S4,2)is normally received.

In this case, at the receiving end, it is known that the high prioritypacket which has been received most recently has the sequence number S4,and the error packet has the sequence number S2 and the retransmissioncount 2.

So, the receiving end sends a retransmission request for the highpriority packet (S2,2), together with the sequence number S4 of the mostrecently received high priority packet as well as the sequence number S2and retransmission count 2 of this packet (S2,2).

Then, at the transmitting end, the sequence number S4 of themost-recently received high priority packet is compared with thesequence number S2 of the error packet for which the retransmissionrequest has been made. In this case, since the sequence number S4 of themost recently received high priority packet is larger than the sequencenumber S2 of the error packet, the transmitting end performs selectiveretransmission for only the error packet.

That is, the high priority packet (S2,3) is transmitted, having, in itsheader, the sequence number S4 and retransmission count 2 of the highpriority packet (S4,2).

Next, still another example of data exchange will be described by usinga sequence diagram of FIG. 14.

Initially, as shown in FIG. 14, at a predetermined transmission timing,the high priority packet (S4,1) is retransmitted, having, in its header,the, sequence number S2 and retransmission count 1 of the high prioritypacket (S2,1) which has been transmitted in advance of this packet, aswell as the sequence number S4 and retransmission count 1 of thispacket.

At the next transmission timing, the next packet (S5) is transmitted,having, in its header, the sequence number 54 and retransmission count 1of the high priority packet (S4,1) as well as the sequence number S5 ofthis packet.

In this example, a transmission error has occurred during the firstpacket transmission, and so the packet (S4,1) has not been received atthe receiving end while only the packet (S5) has been received.

In this state, at the receiving end, it is known that the most-recentlyreceived, high priority packet has the sequence number S1, and the errorpacket has the sequence number S4 and the retransmission count 1.

So, the receiving end sends a retransmission request for the highpriority packet (S4,1), together with the sequence number S1 of themost-recently received high priority packet as well as the sequencenumber S4 and retransmission count 1 of this packet.

Then, at the transmitting end, the sequence number S1 of themost-recently received high priority packet is compared with thesequence number S4 of the error packet for which the retransmissionrequest has been made. In this case, since the sequence number S1 of themost-recently received high priority packet is smaller than the sequencenumber S4 of the error packet the transmitting end performs selectiveretransmission for those packets having sequence numbers larger than thesequence number S1 and equal to or smaller than the sequence number S4.

In this case, the high priority packet (S2,2) is transmitted having, inits header; the sequence number S4 and retransmission count 1 of thehigh priority packet (S4,1) as well as the sequence number S2 andretransmission count 2 of this packet (S2,2) and, subsequently, the highpriority packet (S4,2) is transmitted, having, in its header, thesequence number S2 and retransmission count 2 of the high prioritypacket (S2,2) as well as the sequence number S4 and transmission count 2of this packet.

Thereafter, the packet of sequence number S6 is transmitted having, inits packet, the sequence number S4 and retransmission count 2 of thehigh priority packet (S4,2) as well as the sequence number S6 of thispacket.

Since errors have occurred during transmission of the high prioritypackets (S2,2) and (S4,2), these packets are not received at thereceiving end, and only the packet (S6) is received.

In this state, at the receiving end, it is known that the most-recentlyreceived high priority packet has the sequence number S1, and the errorpacket has the sequence number S4 and the retransmission count 2.

So, the receiving end sends a retransmission request for the highpriority packet (S4,2), together with the sequence number S1 of themost-recently received high priority packet as well as the sequencenumber S4 and retransmission count 2 of this packet.

Then, at the transmitting end, the sequence number S1 of themost-recently received; high priority packet is compared with thesequence number S4 of the error packet for which the retransmissionrequest has been made. In this case, since the sequence number S1 of themost-recently received high priority packet is smaller than the sequencenumber 54 of the error packet the transmitting end performs selectiveretransmission for those packets having sequence numbers which arelarger than the sequence number S1 and equal to or smaller than thesequence number S4.

That is, the high priority packet (S2,3) is transmitted, having, in itsheader, the sequence number S4 and retransmission count 2 of the highpriority packet (S4,2) as well as the sequence number S2 andretransmission count 3 of this packet and, subsequently, the highpriority packet (S4,3) is transmitted, having, in its header, thesequence number S2 and retransmission count 3 of the high prioritypacket (S2,3) as well as the sequence number S4 and retransmission count3 of this packet.

As described above, according to the fourth embodiment, when thesequence number of the most-recently received high priority packet islarger than the sequence number of the error packet, the transmittingend retransmits only the error packet. Therefore, retransmission of theerror packet can be performed efficiently.

In this fourth embodiment, emphasis has been placed on the case whereone data receiving apparatus is provided for one data transmissionapparatus. However, the selective retransmission process of this fourthembodiment can be applied to multicast having a plurality of datareceiving apparatuses for one data transmission apparatus. Hereinafter,a selective retransmission process applicable to multicast will bedescribed as a modification of the fourth embodiment.

[Modification of Embodiment 4]

FIG. 15 is a sequence diagram for explaining a data transmission methodaccording to a modification of the fourth embodiment, illustrating anexample of selective retransmission in the case where a plurality ofdata receiving apparatuses are provided for one data transmissionapparatus.

Initially, as shown in FIG. 15, at a predetermined transmission timing,the high priority packet (S1,1) is transmitted, having, in its header,the sequence number and retransmission count of a previous high prioritypacket as well as the sequence number S1 and retransmission count 1 ofthis packet. In this case, there are two receiving ends for thetransmitting end, i.e., the receiving end 1 and the receiving end 2, andtransmission of the high priority packet (S1,1) is error transmission.

In this case, each receiving end sends a retransmission request for thehigh priority packet (S1,1), together with the sequence number (lastS)of the most-recently received high priority packet.

In response to the retransmission request from the receiving end 1 thetransmitting end retransmits the high priority packet (S1,2). However,after this retransmission, the transmitting end does not performretransmission in response to the retransmission request from thereceiving end 2, because the high priority packet (S1,2) has alreadybeen retransmitted in response to the retransmission request from thereceiving end 2.

As the result of the retransmission of the high priority packet (S1,2),the receiving end 1 receives the high priority packet (S1) while thereceiving end 2 does not receive the high priority packet (S1).

In this case, the receiving end 2 outputs a retransmission request forthe high priority packet (S1,2). In response to this request, thetransmitting end increments the retransmission count and retransmits thehigh priority packet (S1,3).

In the fourth embodiment and the modification thereof, an upper limitmay be set for the retransmission count. Further, the upper limit of theretransmission count may be changed according to the priority value ofthe packet.

[Embodiment 5]

FIGS. 16 and 17 are diagrams for explaining a data transmission methodaccording to a fifth embodiment of the present invention.

According to the data transmission method of this fifth embodiment, inthe data transmission method of the first embodiment absence of asequence number is detected at the receiving end, and a retransmissionrequest for a packet having the absent sequence number is made by usingthis sequence number. At the transmitting end, when the packet of thesequence number for which the retransmission request has been outputfrom the receiving end is stored in the retransmission buffer 17,retransmission of this packet is performed. At the transmitting end,only high priority packets are stored in the retransmission buffer 17.

FIGS. 16(a) and 16(b) are block diagrams illustrating a datatransmission apparatus 105 and a data, receiving apparatus 205,respectively, in a data transmission system which performs real-timedata transmission by the data transmission method of this fifthembodiment.

The data transmission apparatus 105 is identical in structure to thedata transmission apparatus 101 of the first embodiment. The datareceiving apparatus 205 includes a receiving unit 21, an error packetdetection unit 22, a packet decoding unit 23, and a reception historymanagement unit 24, like the data receiving apparatus 201 of the firstembodiment. The receiving unit 21 receives packets transmitted from arelay server (data transmission apparatus at the transmitting end). Theerror packet detection unit 22 detects an error packet in which an errorhas occurred during transmission, in accordance with the output from thereceiving unit 21, and outputs a normal packet which has beentransmitted without a transmission error. The packet decoding unit 23receives the normal packet to decode coded data of the normal packet.The reception history management unit 24 manages the reception historyof packets.

The data receiving apparatus 205 of this fifth embodiment has aretransmission instruction output unit 26 b which outputs aretransmission request for a packet corresponding to the absent sequencenumber detected by the error packet detection unit 22, instead of theretransmission instruction output unit 26 of the data receivingapparatus 201 of the first embodiment.

Next, the function and effect will be described.

FIG. 17 is a sequence diagram for explaining packet selectiveretransmission control in the data transmission method of this fifthembodiment.

When a high priority packet (S1) of sequence number S1, which has beenoutput from the data transmission apparatus (transmitting end) 105, isnot received by the data receiving apparatus (receiving end) 205 due toa transmission error, in the data receiving apparatus 205; the errorpacket detection unit 22 detects that the sequence number S1 is absent,and a retransmission request for the packet of this sequence number S1is output to the transmitting end.

At the transmitting end, it is decided whether the packet of theretransmission request is stored in the retransmission buffer 17 or not.In this case, since the packet of the retransmission request is a highpriority packet, it is stored in the retransmission buffer 17.Therefore, the transmitting end performs retransmission of this packet.

Further, when a packet (S2) of sequence number S2, which has been outputfrom the data transmission apparatus (transmitting end) 105, is notreceived by the data receiving apparatus (receiving end) 205 due to atransmission error, in the data receiving apparatus 205, the errorpacket detection unit 22 detects that the sequence number S2 is absent,and a retransmission request, for the packet of this sequence number S2is output to the transmitting end.

At the transmitting end, it is decided whether the packet of theretransmission request is stored in the retransmission buffer 17 or not.In this case, since the packet of the retransmission request is not ahigh priority packet, it is not stored in the retransmission buffer 17.Therefore, the transmitting end does not perform retransmission of thispacket.

As described above, according to the fifth embodiment of the invention,the receiving end detects the absence of sequence number, and instructsthe transmitting end to retransmit the packet of the absent sequencenumber by using this sequence number. Then, the transmitting endretransmits the packet of the sequence number indicated by theinstruction from the receiving end when this packet is stored in theretransmission buffer. Therefore, the transmission quality in thewireless section in real-time transmission can be improved and,furthermore, the number of retransmission times can be reduced.

[Embodiment 6]

FIGS. 18˜20 are diagrams for explaining a data transmission methodaccording to a sixth embodiment of the present inventions.

In the data transmission method of this sixth embodiment, datatransmission from the transmitting end to the receiving end iscontinuously performed in units of packets each having additionalinformation relating to its sequence number, priority and datareproduction time and, simultaneously, data in the packets received atthe receiving end are successively reproduced. At this time, only thepacket which can arrive at the receiving end within the time limit isretransmitted.

FIG. 18 is a block diagram illustrating a data transmission apparatus106 in a data transmission system which performs real-time datatransmission by the data transmission method of this sixth embodiment.

This data transmission apparatus 106 constitutes a relay server(transmitting end) which relays data transmitted between distributionserver and a terminal 9 (receiving end). The data transmission apparatus106 includes a reproduction time decision unit 39, in addition to theconstituents of the data transmission apparatus 101 of the firstembodiment. The reproduction time decision unit 39 decides a packetwhich cannot arrive at the receiving end within the reproduction time,amongst the packets to be transmitted. In the transmission unit 13 b,the packet decided in the reproduction time decision unit 39 is nottransmitted to the receiving end whether it is stored in theretransmission buffer 17 or not.

Other constituents of the data transmission apparatus 106 are identicalto those of the data transmission apparatus 101 of the first embodiment.

FIG. 19, is a block diagram illustrating a data receiving apparatus 206in the data transmission system which performs real-time datatransmission according to the data transmission method of the sixthembodiment.

The data receiving apparatus 206 includes a reproduction time decisionunit 43, instead of the packet priority decision unit 25 of the datareceiving apparatus 201 of the first embodiment. The reproduction timedecision unit 43 detects the reproduction time which is given to theerror packet detected by the error packet detection unit 22 and thearrival time of the error packet at the receiving end, decides thearrival time limit based on the reproduction time, and decides whetheror not the error packet has arrived at the receiving end before thearrival time limit. On the basis of the result of the decision, theretransmission instruction output unit 26 instructs the transmitting endto retransmit the error packet which has arrived at the receiving endbefore the arrival time limit, by using the sequence number of thepacket. The decision of the arrival time limit by the reproduction timedecision unit 43 is performed based on at least one of the allowablepacket delay time which is decided at the receiving end, and the packettransmission delay time between the transmitting end and the receivingend.

Other constituents of the data receiving apparatus 206 are identical tothose of the data receiving apparatus 201 of the first embodiment.

As for the method of deciding the arrival time limit, it will bedescribed in detail as an eighth embodiment of the present invention.

Next, the function and effect will be described.

FIG. 20 is a sequence diagram for explaining packet retransmissioncontrol with time limit, in the data transmission method according tothis sixth embodiment.

In the data transmission apparatus (transmitting end) 106, each packetto be transmitted is given a reproduction time at the receiving end.

For example, when high priority packets (S1) and (S2) having sequencenumbers S1 and S2, which have been transmitted from the datatransmission apparatus (transmitting end) 106, are not normally receivedat the data receiving apparatus (receiving end) 206 due to atransmission error, in the data receiving apparatus 206, the errorpacket detection unit 22 decides that these high priority packets (S1)and (S2) are error packets. Further, the reproduction time decision unit43 detects the reproduction times (T1) and (T2) and the arrival times ofthese error packets, decides the arrival time limits (T1+α) and (T2+α)in accordance with the reproduction times, and decides whether or notthese error packets have arrived at the receiving end before the arrivaltime limits, respectively.

Since the high priority padket (S1) has arrived at the receiving endbefore the time limit, the receiving end instructs the transmitting endto retransmit this packet.

On the other hand, since the high priority packet (S2) has not arrivedat the receiving end before the time limit, the receiving end does notinstruct the transmitting end to retransmit this packet.

In the data transmission apparatus 106, when the data of the packethaving the sequence number indicated by the retransmission request fromthe receiving end, is stored in the retransmission buffer, the data ofthe packet the transmission time of which does not pass the reproductiontime is retransmitted to the receiving end. On the other hand, as forthe data of the packet the transmission time of which has passed thereproduction time, this data is discarded without being transmitted, andthe data in the retransmission buffer is discarded successively from thepacket which cannot arrive at the receiving end within the datareproduction time.

As described above, according to the sixth embodiment of the presentinvention, data transmission from the transmitting end to the receivingend is continuously performed in units of packets each having additionalinformation relating to its sequence number, priority, and datareproduction time at the receiving end and, simultaneously, data of thepackets received at the receiving end are successively reproduced. Atthis time, only the packets which have arrived at the receiving endwithin the time limit at the receiving end, are retransmitted to thetransmitting end. Therefore, the transmission quality of wirelesssections in real-time transmission is improved and, moreover, the numberof retransmission times can be reduced.

While in this sixth embodiment emphasis has been placed on the methodfor recovering errors of data in packets there is a case whereadditional information added to the packet header (i.e., the sequencenumber, the reproduction time, etc.) has an error. There are variousmethods of recovering the additional information as well as, recoveringthe errors in data. Hereinafter, these methods will be described asmodifications of the sixth embodiment.

[Modification 1 of Embodiment 6]

In a first modification of the sixth embodiment, in addition to theprocesses at the transmitting end and the receiving end in the datatransmission method of the, sixth embodiment, at the transmitting end,error correction codes for the sequence number and the reproduction timeare given to the packet to be transmitted. At the receiving end, errorcorrection for the sequence number and the reproduction time isperformed by utilizing the error correction codes and, thereafter, errorpacket retransmission control is performed on the basis of the decidedarrival limit time.

In the first modification of the sixth embodiment so constructed, evenwhen the sequence number and the reproduction time of the error packethave errors, these additional information can be corrected.

[Modification 2 of Embodiment 6]

In a second modification of the sixth embodiment, in addition to theprocesses at the transmitting end and the receiving end in the datatransmission method of the sixth embodiment, at the transmitting end,the sequence number and the reproduction time of a predetermined packetare embedded in a packet to be transmitted next. At the receiving end,the embedded information (i.e., the sequence number and the reproductiontime) of the previous packet (the predetermined packet) is extractedwhen the next packet is received. On the basis of this information, thearrival time limit of the predetermined packet is decided; andretransmission control is performed for the predetermined packet as anerror packet.

FIG. 21 is a sequence diagram for explaining packet data retransmissioncontrol with time limit, in a data communication system according to thesecond modification of the sixth embodiment.

At the transmitting end, a predetermined packet is given the sequencenumber and the reproduction time at the receiving end, and the sequencenumber and the reproduction time of this predetermined packet are alsoembedded in a next packet which is to be transmitted next to thepredetermined packet.

Thereafter, as shown in FIG. 21, packets (S1) and (S2) of sequencenumbers S1 and S2 are transmitted from the transmitting end, and thepacket (S1) is not normally received at the receiving end due to atransmission error. Even in this case, as long as the packet (S2) isnormally received at the receiving end, the receiving end can detect thesequence number S1 and the reproduction time, (T1) of the packet (S1)previous to the packet (S2).

So, at the receiving end, the arrival time limit (T1+α) is decided onthe basis of the reproduction time (T1) of the error packet (S1), and itis decided whether or not the receiving end can receive theretransmitted packet within the arrival time limit if a retransmissionrequest is made at this point of time. In this case, there is apossibility that the packet (S1) will arrive at the receiving, endbefore the time limit if a retransmission request for the packet (S1) ismade when receiving the packet (S2) and, therefore, the receiving endsends a retransmission request for the packet (S1) to the transmittingend.

Thereafter, as shown in FIG. 21, packets (S3) and (S4) of sequencenumbers S3 and S4 are transmitted from the transmitting end, and thepacket (S3) is not normally received at the receiving end due to atransmission error. Even in this case, as long as the packet (4) isnormally received at the receiving end, the receiving end can detect thesequence number S3 and the reproduction time (T3) of the packet (S3)previous to the packet (S4).

So, at the receiving end, the arrival time limit (T3+α) is decided onthe basis of the reproduction time (T3) of the error packet (S3), andit, is decided whether the packet (S3) has arrived at the receiving endbefore the time limit or not. In this case, since the packet (S3) hasnot arrived at the receiving end before the time limit, the receivingend does not instruct the transmitting end to retransmit the packet(S3).

In the second modification of the sixth embodiment so constructed, atthe transmitting end, the sequence number and the reproduction time of apredetermined packet are also embedded in a packet to be transmittednext to the predetermined packet. Therefore, even when errors occur innot only the data of the packet but also the sequence number and thereproduction time, the receiving end can make a retransmission requestfor the error packet. Moreover, since the receiving end does not make aretransmission request for the packet which cannot arrive at thereceiving end within the reproduction time at the receiving end, thenumber of retransmission times can be reduced.

As still another modification, the selective retransmission controlaccording to any of the first to fifth embodiments and theirmodifications may be combined with the time-limit retransmission controlaccording to any of the sixth embodiment and its modifications.

For example, the second embodiment in which each packet is given errorcorrection codes for its sequence number and priority information may becombined with the first modification of the sixth embodiment in whicheach packet is given error correction codes for its sequence number andreproduction time (first combination). According to this combination,each packet is given error correction codes for its sequence number,priority information, and reproduction time.

Further, the third embodiment in which the sequence number and thepriority information of a predetermined packet are embedded in a packetto be transmitted next to the predetermined packet, may be combined withthe second modification of the sixth embodiment in which the sequencenumber and the reproduction time of a predetermined packet are embeddedin a packet to be transmitted next to the predetermined packet (secondcombination). According to this combination, the sequence number, thepriority information, and the reproduction time of the predeterminedpacket are embedded in the packet to be transmitted next to thepredetermined packet.

Further, the modification of the third embodiment in which the processof embedding the sequence number and the priority information of a highpriority packet to be transmitted into subsequent packets is continueduntil a next high priority packet is transmitted, may be combined withthe second modification of the sixth embodiment in which the sequencenumber and the reproduction time of a predetermined packet are embeddedin a packet to be transmitted next to the predetermined packet (thirdcombination). According to this combination, the process of embeddingthe sequence number, the priority information, and the reproduction timeof a high priority packet into subsequent, packets is continued until anext high priority packet is transmitted.

Furthermore, according to the combination of the selectiveretransmission control and the retransmission control with time limit(second or third combination), the quantity of data to be embedded inthe packet increases. Therefore, there is proposed a method of embeddinga difference between the header information (sequence number, priorityinformation, reproduction time) of a predetermined packet and the headerinformation of a subsequent packet, in the subsequent packet.

Hereinafter, this method will be described as a third, modification ofthe sixth embodiment.

[Modification 3 of Embodiment 6]

FIG. 22 is a sequence diagram for explaining packet, retransmissioncontrol with time limit, in the data transmission method according to athird modification of the sixth embodiment.

At the transmitting end, in a packet to be transmitted next to apredetermined packet, difference information relating to the sequencenumbers and thee reproduction time of the predetermined packet which haspreviously been transmitted is embedded together with the sequencenumber and the reproduction time of this packet. This differenceinformation is composed of a difference between the sequence number ofthe predetermined packet and the sequence number of the next packet(sequence difference) and a difference between the reproduction time ofthe predetermined packet and the reproduction time of the next packet(time difference).

For example, as shown in FIG. 22, packets (S1) and (S2) of sequencenumbers S1 and S2 are transmitted from the transmitting end, and thepacket (S1) is not normally received at the receiving end due totransmission error. Even in this case, as long as the packet (S2) isnormally received at the receiving end, the receiving end can obtain thesequence number S1 and the reproduction time (T1) of the packet (S1)previous td the packet (S2).

That is, the sequence number S1 of the packet (S1) can be obtained bysubtracting the sequence difference (S2−S1) from the sequence number S2of the packet (S2). Further, the reproduction time (T1) of the packet(S1) can be obtained by subtracting the difference time (T2−T1) from thereproduction time (T2) of the packet (S2).

Therefore, at the receiving end, the arrival time limit (T1+α) isdecided on; the basis of the reproduction time (T1) of the error packet(S1), and it is decided whether this packet has arrived at the receivingend before the arrival time limit or not. For example, when the packet(S1) has arrived at the receiving end before the arrival time limit, thereceiving end instructs the transmitting end to retransmit the packet(S1), by using the sequence number S1.

In the third modification of the sixth embodiment so constructed, sincea difference between the header information (sequence number priorityinformation, reproduction time) of a predetermined packet and the headerinformation of a subsequence packet is embedded in the subsequencepacket, the quantity of information to be embedded in the packet isreduced.

While in the third modification of the sixth embodiment the sequencenumber and the reproduction time are described as information to beembedded in the subsequent packet, information to be embedded is notrestricted thereto. For example, in addition to the sequence number andthe reproduction time, the retransmission count and the priority may beembedded.

Hereinafter, a description will, be given of embodiments relating toapplication sections of the retransmission control of the datatransmission methods according to the aforementioned embodiments.

[Embodiment 7]

In a data transmission method according to a seventh embodiment of theinvention, data transmission between a distribution server and aterminal is per formed in packet units through a relay server and, atthe terminal end, data of received packets are successively reproduced.When a transmission error occurs between the relay server and theterminal, the relay server performs retransmission of the error packetin accordance with a retransmission request from the terminal. When atransmission error occurs between the distribution server and the relayserver, the distribution server performs retransmission of the errorpacket in accordance with a retransmission request which is transmittedfrom the terminal through the relay server.

FIG. 23 is a block diagram for explaining a data transmission methodaccording to this seventh embodiment, illustrating a data transmissionapparatus 107 in a system performing data transmission according to thedata transmission method.

This data transmission apparatus 107 constitutes a relay server(transmitting end) which relays data transmitted between thedistribution server and the terminal (receiving end), and it includes aretransmission instruction output unit 51, in addition to theconstituents of the data transmission apparatus 101 of the firstembodiment. The retransmission instruction output unit 51 instructs thedistribution server to retransmit an error packet for which aretransmission request has been output from the terminal, on the basisof the result of decision in the retransmission propriety decision unit16. Other constituents of the data transmission apparatus 107 areidentical to those of the data transmission apparatus 101 of the firstembodiment.

Next, the function and effect will be described.

In the seventh embodiment so constructed, when data transmission betweenthe distribution server and the terminal is performed in packet units,the data transmission apparatus 107 relays data between the distributionserver and the terminal.

When a transmission error occurs between the relay server (datatransmission apparatus 107) and the terminal, the relay server performsretransmission of the error packet. In this case, a retransmissionrequest for the error packet from the terminal is not relayed to thedistribution server.

Further, when a transmission error occurs between the distributionserver and the relay server (data transmission apparatus 107), aretransmission request for the error packet from the terminal istransmitted to the distribution server, and the error packet which isretransmitted from the distribution server is relayed by the datatransmission apparatus 107 to be transmitted to the terminal.

In the seventh embodiment so constructed, when a transmission erroroccurs between the relay server (the data transmission apparatus) andthe terminal, a retransmission request for the error packet from theterminal is not relayed to the distribution server, and the relay serverperforms retransmission for the error packet. Therefore, the number ofretransmission times between the distribution server and the relayserver can be reduced.

[Modification of Embodiment 7]

A modification of the seventh embodiment is a data transmission methodin which data transmission between a distribution server and a terminalis performed in packet units through a relay server and, at the terminalend, data of received packets are successively reproduced. When atransmission error occurs between the distribution server and the relayserver, the error packet is not transmitted to the terminal, but therelay server transmits a retransmission request to the distributionserver and the error packet is retransmitted from the distributionserver to the relay server.

FIG. 24 is a block diagram illustrating a data transmission apparatus107 a in a data communication system according to the modification ofthe seventh embodiment.

The data transmission apparatus 107 a constitutes a relay server(transmitting end) which relays data transmitted between a distributionserver and a terminal (receiving end), and it includes an error packetdecision unit 22, a reception history management unit, 52, and aretransmission instruction output unit 51 a, in addition to theconstituents of the data transmission apparatus 101 of the firstembodiment. The error packet decision unit 22 decides whether eachpacket received is an error packet or not on the basis of the outputfrom the receiving unit 11 which receives packets from the distributionserver, and outputs normally received packets. The reception historymanagement unit 52 manages the history of the normally received packets.The retransmission instruction output unit 51 a instructs thedistribution server to retransmit the error packet, according to theresult of the decision in the retransmission decision unit 16, theresult of the detection in the error packet detection unit 22, and thecontent s of the reception history in the reception history managementunit 52. Other constituents of the data transmission apparatus 107 a areidentical to those of the data transmission apparatus 101 of the firstembodiment.

Next, the function and effect will be described.

In the modification of the seventh embodiment so constructed, when datatransmission between the distribution server and the terminal isperformed in packet units, the data transmission apparatus 107 a relaysthe data between the distribution server and the terminal.

When a transmission error occurs between the relay server (datatransmission apparatus 107 a) and the terminal, the relay serverperforms retransmission of the error packet. In this case, aretransmission request for the error packet from the terminal is notrelayed to the distribution server.

Further, when a transmission error occurs between the distributionserver and the relay server (data transmission apparatus 107 a) theerror packet is not transmitted to the terminal, but a retransmissionrequest from the relay server is transmitted to the distribution serverand the error packet is retransmitted from the distribution server tothe relay server.

Then, the relay server transmits the error packet retransmitted from thedistribution server, to the terminal.

In the modification of the seventh embodiment, when a transmission erroroccurs between the distribution server and the relay server, the errorpacket is not transmitted to the terminal, but the relay server outputsa retransmission request to the distribution server and thereby thedistribution server retransmits the error packet to the relay server.Therefore, wasteful transmission of the error packet from the relayserver to the terminal is avoided.

[Embodiment 8]

FIGS. 25˜27 are diagrams for explaining a data transmission methodaccording to an eighth embodiment of the present invention.

In this data transmission method, data transmission between adistribution server and a terminal is performed in packet units and, atthe terminal end, data of received packets are successively reproduced.Further, information about a reproduction delay time which is allowablefor the data reproduction time at the terminal end, is transmitted, fromthe terminal to the distribution server. In the distribution server, thereproduction time at the terminal is estimated on the basis of thereproduction delay time and the transmission delay time which isrequired for data transmission between the distribution server and theterminal.

FIG. 25 is a block diagram illustrating a data transmission apparatus108 in a data communication system according, to this eighth embodimentof the invention.

This data transmission apparatus 108 constitutes a relay server(transmitting end) which relays data transmission between a distributionserver and a terminal (receiving end), and it includes a receiving unit11, a transmission queue management unit 12, and a transmission unit 13.The receiving unit 11 receives packets transmitted from the distributionserver. The transmission queue management unit 12 sets the transmissionorder of the received packets, in accordance with additionalinformation, such as a sequence number, which is given to each packet.The transmission unit 13 transmits the packets in the transmission orderwhich is set by the unit 12.

The data transmission apparatus 108 further includes a time stampextraction unit 75 and an allowable reproduction delay informationreceiving unit 72. The time stamp extraction unit 75 extracts a timestamp which is given to each packet as time information, through thetransmission unit 13. The allowable reproduction delay informationreceiving unit 72 receives information relating to the allowablereproduction delay time (arrival time limit) from the receiving end(terminal).

Further, the data transmission apparatus 108 includes a transmissiondelay measurement packet transmitting and receiving unit 71, and atransmission delay measurement unit 73. The transmission delaymeasurement packet transmitting and receiving unit 71 performstransmission and reception of a packet used for measuring a transmissiondelay (transmission delay measurement packet). The transmission delaymeasurement unit 73 measures a transmission delay in accordance with thetime required for forward and backward transmission of the transmissiondelay measurement packet between the transmitting end and the receivingend.

Further, the data transmission apparatus 108 includes a reproductiontime calculation unit 74 which estimates the data reproduction time atthe receiving end, in accordance with the transmission delay, the timestamp, and the allowable reproduction delay time (arrival time limit),and outputs the estimated reproduction time.

Furthermore, like the data transmission apparatus according toembodiment 1 or 6, the data transmission apparatus 108 has the functionof storing a packet the priority of which is equal to or higher than apredetermined value in a retransmission buffer and discarding a packetwhich cannot be in the time for retransmissions and the function ofretransmitting a packet for which a retransmission request is outputfrom the receiving end, although these functions are not shown in thefigure.

To be specific, the data transmission apparatus 108 includes theconstituents corresponding to the retransmission instruction receivingunit 14, the packet priority decision unit 15, the retransmissiondecision unit 16, the retransmission buffer 17, and the retransmissionbuffer management unit 18 which are included in the data transmissionapparatus 101 of the first embodiment. In the transmission queuemanagement unit 12, setting of the packet transmission order isperformed on all the packets to be transmitted, including not only thereceived packets but also the packets to be transmitted.

FIG. 26 is a block diagram illustrating a data receiving apparatus 208in the data transmission system which performs real-time datatransmission by the data transmission method according to this eighthembodiment.

The data receiving apparatus 208 includes a receiving unit 21 whichreceives packets transmitted from the relay server (data transmissionapparatus at the transmitting end), and a packet decoding unit 23 whichdecodes the received packets.

The data receiving apparatus 208 further includes a transmission delaymeasurement packet transmitting and receiving unit 62, an allowablereproduction delay decision unit 63, and an allowable reproduction delayinformation transmitting unit 61. The transmission delay measurementpacket transmitting and receiving unit 62 receives the transmissiondelay measurement packet which has been transmitted from thetransmission delay measurement packet transmitting and receiving unit 71at the transmitting end, and returns it to the transmitting end. Theallowable reproduction delay decision unit 63 decides an allowablereproduction delay time, and outputs information relating to this. Theallowable reproduction delay information transmitting unit 61 transmitsthe information relating to the allowable reproduction delay time, tothe transmitting end.

Furthermore, like the data receiving apparatuses according toembodiments 1 and 6, the data receiving apparatus 208 has the functionof detecting error packets and outputting a retransmission request foran error packet which can be in the time for retransmission, althoughthis is not shown in the figure.

To be specific, the data receiving apparatus 208 includes theconstituents corresponding to the error packet detection unit 22, thereception history management unit 24, the packet priority decision unit25, and the retransmission instruction output unit 26 which are includedin the data receiving apparatus 201 of the first embodiment.

Next, the function and effect will be described.

FIGS. 27(a) and 27(b) are diagrams for explaining a method forestimating the data reproduction time at the terminal, in the serveraccording to this eighth embodiment. More specifically, FIG. 27(a) showsthe relationship between the packet time stamp and the packet outputtime (reproduction time) or the like, and FIG. 27(b) shows a method formapping the time stamp to the standard time.

In the data transmission apparatus (transmitting end) 108, when thetransmission unit 13 performs packet transmission in accordance with thetransmission order which is set by the transmission queue managementunit 12, the time stamp t_(p) included in each packet is extracted bythe time stamp extraction unit 75, and output to the reproduction timecalculation unit 74. Further, the allowable reproduction delay timed_(p) which is decided by the decision unit 63 of the data receivingapparatus (receiving end) 208 and transmitted through the transmissionunit 61, is received by the allowable reproduction delay informationreceiving unit 72 and output to the reproduction time calculation unit74. Further, the transmission delay measurement packet is transmittedfrom the, transmission delay measurement packet transmitting andreceiving unit 71 at the transmitting end to the transmission delaymeasurement packet transmitting and receiving unit 62 at the receivingend, sand the transmission delay measurement packet is returned from thetransmitting and receiving unit 62 at the receiving end to thetransmitting and receiving unit 72 at the transmitting end, whereby theserver-to-terminal transmission delay d_(s) is obtained by thetransmission delay measurement unit 73 and output to the reproductiontime calculation unit 74. The time when each packet is output from theserver is measured at the transmitting end.

Then, the reproduction time calculation unit 74 estimates thereproduction time (packet output time) t_(o) of each packet at thereceiving end.

To be specific, as shown in FIG. 27(a), the packet output time t_(o) isobtained by adding the allowable reproduction delay time d_(p) to thetime stamp t_(p), and the terminal reception time t_(r) is obtained byadding the server-to-terminal transmission delay d_(s) to the serveroutput time t_(s). Accordingly, the reproduction time calculation unit74 obtains the packet output time t_(o) by (t_(p)+d_(p)) and theterminal reception time t_(r) by (t_(s)+d_(s)).

Thereafter, in the data transmission apparatus 108, on the basis ofcomparison between (t_(s)+d_(s)) and (t_(p)+d_(p)) it is decided whetherthe transmission packet can arrive at the receiving end before thereproduction time or not, i.e., whether the relationship, terminalreception time t_(r)≦packet output time t_(o), is satisfied or not.Based on the result of this decision, retransmission of the error packetrequested, and discard of the packet in the retransmission buffer areperformed.

While in this eighth embodiment the reproduction delay d_(p) istransmitted from the receiving terminal to the server, this may be afixed value. Further, temporal synchronization between the server andthe terminal is made by, for example NTP (network time protocol) ofInternet standard.

Further, the time stamp t_(p) of each packet is obtained from the timestamp of the RTP header, this time stamp value must be mapped onto thesame, time axis as the server output time or the like.

Hereinafter, the RTP time stamp mapping method in the relay server willbe described briefly.

Since the RTP time stamp is not expressed directly by the standard time,it is mapped to time information based on the standard time, by usinginformation included in other standard protocols RTCP. (Real-TimeControl Protocol) and RTSP (Real-Time Stream Protocol). The relay server(data transmission apparatus) 108 must know this mapping informationand, therefore, the relay, server 108 obtains this mapping informationwhen it relays the mapping information.

For example, as shown in FIG. 27(b), when the mapping information forthe time stamps included in the Internet standard protocols RTCP andRTSP, which is output from the server S, is relayed by the relay serverIS to the terminal T, the mapping information is analyzed by the relayserver IS. Further, the mapping information for the time stamps at theterminal is transmitted from the terminal to the server.

As described above, according to the eighth embodiment, since the server(data transmission apparatus) estimates the actual output time(reproduction time) of each packet at the terminal, a packet whichcannot be in time for reproduction can be eliminated from theretransmission buffer.

In this eighth embodiment, the method for deciding the transmissiondelay time and the allowable reproduction delay time is not restrictedto the above-mentioned method. For example, during data reproduction atthe receiving end, the transmission delay between the server and theterminal may be dynamically updated according to the information fromthe receiving end. Alternatively, during data reproduction at thereceiving end, the allowable reproduction delay may be measured andchanged dynamically.

Furthermore, while in this eighth embodiment the actual output time(reproduction time) of each packet at the terminal is estimated in therelay server (data transmission apparatus), it may be estimated in thedistribution server in accordance with the allowable reproduction delaytime from the terminal or the transmission delay time required for datatransmission between the distribution server and the terminal.

[Embodiment 9]

FIGS. 29˜31 are diagrams for explaining a data transmission methodaccording to a ninth embodiment of the present invention. In the datatransmission method of this ninth embodiment, data transmission from thetransmitting end to the receiving end is continuously performed in unitsof packets each having additional information relating to its sequencenumber, priority, and data reproduction time at the receiving end, whichare required to realize real-time transmission in packet units, whilesuccessively reproducing data of the packets received at the receivingend. At this time, only error packets the priorities of which are equalto or higher than, a predetermined value are retransmitted.

FIG. 29(a) is a block diagram illustrating a data transmission apparatus109 in a data transmission system which performs real-time datatransmission according to the data transmission method of this ninthembodiment.

The data transmission apparatus 109 constitutes a relay server(transmitting end) which relays data transmitted between a distributionserver and a terminal (receiving end). The data transmission apparatus109 includes a receiving unit 11, a transmission queue management unit12, and a transmission unit 13. The receiving unit 11 receives packetstransmitted from the distribution server. The transmission queuemanagement unit 12 sets the transmission order of the received packetsand the packets to be retransmitted (retransmission packets) inaccordance with the above-described additional information. Thetransmission unit 13 transmits the packet data in the transmission orderwhich has been set by the transmission queue management unit 12.

The data transmission apparatus 109 further includes a high prioritysequence number management unit 81, a sequence number correspondencemanagement unit 82, and a high priority sequence number insertion unit83. When the priority of a packet transmitted by the transmission unit13 is equal to or higher than a predetermined value, the high prioritysequence number management unit 81 increments the value of the sequencenumber which corresponds to only the high priority packet (high prioritysequence number), and stores the incremented value. The sequence numbercorrespondence management unit 82 stores the correspondence between thevalue of the sequence number of the transmitted high priority packet andthe value of the incremented high priority sequence number of this highpriority packet. The high priority, sequence insertion unit 83 outputsthe value of the high priority sequence number of each high prioritypacket, which is stored in the management unit 81, so that it isinserted in the packet to be transmitted.

The high priority sequence numbers managed by the high priority sequencenumber management unit 81 correspond to the number of high prioritypackets transmitted from the transmitting end.

Further, the data transmission apparatus 109 includes a retransmissionbuffer 17, a packet priority decision unit 15, and a retransmissionbuffer management unit 18. The retransmission buffer 17 storespredetermined packets amongst the received packets, as retransmissionpackets. The packet priority decision unit 15 decides the priorities ofthe received packets. The retransmission buffer management unit 18controls the retransmission buffer 17 such that data of packets thepriorities of which are equal to or higher than a predetermined valueare stored in the buffer 17, in accordance with the decided prioritiesof the packets.

Further, the data transmission apparatus, 109 includes a retransmissioninstruction receiving unit 14 and a retransmission decision unit 16 c.The retransmission instruction receiving unit 14 receives aretransmission request indicating a high priority sequence number, fromthe terminal at the receiving end. The retransmission decision unit 16 cdecides whether retransmission of a packet for which the retransmissionrequest has been made is performed or not. The retransmission decisionunit 16 retrieves the management information in the sequence numbercorrespondence management unit 82, in accordance with the high prioritysequence number indicated by the retransmission request, to obtain thesequence number corresponding to the high priority sequence number usedfor the retransmission request, and decides that the requested packet isto be retransmitted, only when the packet of the sequence number isstored in the retransmission buffer 17.

In FIG. 29(a), real-time data transmission is performed in packet unitsbetween the distribution server and the terminal through the relayserver or the like, and the data transmission apparatus 109 constitutesthe relay server. However, the relay server may serve as thedistribution server. To be specific, when the data transmissionapparatus serves as the distribution server, it is constructed as shownin FIG. 29(b). In FIG. 29(b), the receiving unit 11 of the datatransmission apparatus 109 is replaced with a coded packet generationunit 10 a which encodes the data and outputs the coded data in packetunits, and a priority allocation unit 10 b which allocates additionalinformation, such as a priority, to each packet output from the codedpacket generation unit 10 a.

FIG. 30 is a block diagram illustrating a data receiving apparatus 209in the data transmission system which performs real-time datatransmission according to the data transmission method of the ninthembodiment.

The data receiving apparatus 209 includes a receiving unit 21, an errorpacket detection unit 22 a, and a packet decoding unit 23. The receivingunit 21 receives the packets transmitted from the data transmissionapparatus at the transmitting end. The error packet detection unit 22 adetects error packets in which errors have occurred during transmissionand outputs normal packets which have been transmitted withouttransmission errors. The packet decoding unit 23 receives the normalpackets and decodes the coded data of the normal packets.

When the high priority sequence number inserted in the packet from thedata transmission apparatus (transmitting end high priority sequencenumber) is correctly extracted, the error packet detection unit 22 aoutputs both of the value of the transmitting end high priority sequencenumber and the value of the receiving end high priority sequence numberat this point of time. The value of the receiving end high prioritynumber corresponds to the number of the high priority packets receivedat the receiving end, and this value is incremented every time a highpriority packet is received at the receiving end.

Further, the data receiving apparatus 209 includes a high prioritysequence number management unit 91 and are transmission sequence numberdecision unit 92. When the error packet detection unit 122 a outputs anormal packet, the high priority sequence number management unit 91increments the value of the receiving end high priority sequence numberand stores it. The retransmission sequence number decision unit 92compares the value of the transmitting end high priority sequence numberoutput from the error packet detection unit 22 a with the value of thereceiving end high priority sequence number. When these values are notequal, the decision unit 92 outputs the values ranging from the valueobtained by adding 1 to the value of the receiving end high prioritysequence number to the value of the transmitting end high prioritysequence number, as the values of retransmission sequence numbers(transmitting end high priority sequence numbers).

The high priority sequence number management unit 91 increments thevalue of the stored receiving end high priority sequence number everytime the retransmission sequence number decision unit 92 outputs a highpriority sequence number.

Further, the data receiving unit 209 includes a retransmissioninstruction output unit 26 c which outputs a retransmission request foran error packet to the transmitting end, on the basis of thetransmitting end high priority sequence number which is output as aretransmission sequence number from the retransmission sequence numberdecision unit 92.

Next, the function and effect will be described.

FIG. 31 is a sequence diagram for explaining the selective packetretransmission control in the data transmission method of this ninthembodiment.

In the description with respect to FIG. 31, a sequence number [S+n]indicates a sequence number having a value “S+n”, a sequence number[H+n], indicates a high priority sequence number having a value “H+n”,and a packet (S+n) indicates a packet having a sequence number the valueof which is “S+n”. Further, n is any of 0, 1, 2, 3, 4, and 5.

For example, assuming that the priorities equal to or higher than thepredetermined value are high priorities while the priorities lower thanthe predetermined value are low priorities, as shown in FIG. 31, when anerror has occurred during transmission of high priority packets (S+1)and (S+2) of sequence numbers [S+1] and [S+2], retransmission requestsfor these high priority packets are made at the receiving end. However,when an error has occurred during transmission of a low priority packet(S+3) of sequence number [S+3], no retransmission request is made forthis low priority packet (S+3).

To be specific, each packet transmitted from the distribution server isgiven additional information relating to its sequence number andpriority, in the data transmission apparatus 109 as a relay server, thetransmission order of the received packets is set by the transmissionqueue management unit 12, and then the packets are supplied to thetransmission unit 13. On the other hand, the priorities of the receivedpackets are decided by the packet priority decision unit 15. Then, inthe transmission unit 13, transmission of these packets is performedaccording to the transmission order which has been set. Further, thosepackets the priorities of which are decided as being equal to or higherthan the predetermined value are stored in the retransmission buffer 17under control of the retransmission buffer management unit 18. Further,in the retransmission buffer 17, data are successively released(discarded) from the packets which cannot be in time for reproduction,under control of the management unit 18.

Next, a description will be given of management of the sequence numbersat the time of packet transmission.

When transmitting a high priority packet, the high priority sequencenumber is incremented.

To be specific, when a high priority packet (S+0) of sequence number[S+0] is transmitted by the transmission unit 13, the value of thetransmitting end high priority sequence number [H+0] which is stored inthe high priority sequence number management unit 81 is incremented to“H+1”. At this time, the value of the sequence number [S+0] of the highpriority packet (S+0) and the incremented value of the transmitting endhigh priority sequence number [H+1] are entered, by one-to-onecorrespondence, in the sequence number correspondence management unit82.

Likewise, when a high priority packet (S+1) of sequence number [S+1] istransmitted by the transmission unit 13, the value of the transmittingend high priority sequence number [H+1] which is stored in the highpriority sequence number management unit 81 is incremented to “H+2”. Atthis time, the value of the sequence number [S+1] of the high prioritypacket (S+1) and the incremented value of the transmitting end highpriority sequence number [H+2] are entered, by one-to-onecorrespondence, in the sequence number correspondence management unit82.

Further, also when a high priority packet (S+2) is transmitted, like thehigh priority packets (S+0) and (S+1), the transmitting end highpriority sequence number [H+2] in the high priority sequence numbermanagement unit 81 is incremented, and the sequence number [S+2] of thehigh priority packet (S+2) and the incremented transmitting end highpriority sequence number [H+3] are entered, by one-to-onecorrespondence, in the sequence number correspondence management unit82.

On the other hand, when transmitting a low priority packet, thecorresponding low, priority sequence number is not incremented

To be specific, when a low priority packet (S+3) of sequence number[S+3] is transmitted by the transmission unit 13, the value of thetransmitting end high priority sequence number [H+3] stored in the highpriority sequence number management unit 81 is not updated butmaintained as it is. At this time, the process of entering the sequencenumber of the transmitted packet and the transmitting end high prioritysequence number [H+3] which is stored in the high priority sequencenumber management unit 81, in the sequence number correspondencemanagement unit 82, is not performed.

Next, a description will be given of a sequence number embedding processat the time of packet transmission.

When the high priority packet (S+1) is transmitted, the value of thetransmitting end high priority sequence number [H+] which is stored inthe high priority sequence number management unit 81 at this point oftime, is embedded in the header of the transmission packet (S+1) by thehigh priority sequence number insertion unit 83. Likewise, when the lowpriority packet (S+3) is transmitted, the value of the transmitting endhigh priority sequence number [H+3] which is stored in the high prioritysequence number management unit 81 at this point of time, is embedded inthe header of the transmission packet (S+3) by the high prioritysequence number insertion unit 83. Thereafter, the transmission packethaving the transmitting end high priority sequence number so embedded inits header, is transmitted to the receiving end by the transmission unit13.

Those packets output from the distribution server are successivelytransmitted to the terminal (data receiving apparatus) 209 through therelay server.

In the data receiving apparatus 209, the packets from the relay server(data transmission apparatus) 109 are received by the receiving unit 21,and the received packets are supplied to the error packet detection unit22 a. The normally received high priority packet (S+0) is output fromthe error packet detection unit 22 a to the packet decoding unit 23, andthe value of its receiving end high priority sequence number (i.e., thevalue of the high priority sequence number [H+0] stored in the highpriority sequence number management unit 91) is incremented to “H+1”.

It is assumed that a transmission error has occurred during transmissionof the high priority packets (S+1) and (S+2) and, thereafter, the lowpriority packet (S+3) subsequent to these packets has been transmittedwithout a transmission error.

In this case, the normally transmitted low priority packet (S+3) isoutput from the error packet detection unit 22 a to the packet decodingunit 23, but the value of the receiving end high priority sequencenumber [H+1] which is stored in the high priority sequence numbermanagement unit 91 is not incremented.

Further, in the error packet detection unit 22 a, when the transmittingend high priority sequence numbers [H+0] and [H+3] which are inserted inthe high priority packet (S+0) and the low priority packet (S+3),respectively, are correctly extracted, these transmitting end highpriority sequence, numbers [H+0] and [H+3] are output to theretransmission sequence number decision unit 92. Further, the receivingend high priority sequence numbers [H+0] and [H+1] which are stored inthe high priority sequence number management unit 91 at the time whenthe transmitting end high priority sequence numbers [H+0] and [H+3] areextracted by the error packet detection unit 22 a, are output to theretransmission sequence number decision unit 92.

For example, at the time when the transmitting end high prioritysequence number [H+0] is extracted, the value [H+0] of the receiving endhigh priority sequence number stored in the high priority sequencenumber management unit 91 as well as the transmitting end high prioritysequence number [H+0] are output to the retransmission sequence numberdecision unit 92. At the time when the transmitting end high prioritysequence number [H+3] is extracted, the value [H+1] of the receiving endhigh priority sequence number stored in the high priority sequencenumber management unit 91 as well as the transmitting end high prioritysequence number [H+3] are output to the retransmission sequence numberdecision unit 92.

In the retransmission sequence number decision unit 92, the transmittingend high priority sequence number and the receiving end high prioritysequence number, which have been supplied at the same time, arecompared, to decide whether retransmission is to be requested to thetransmitting end.

For example, as the result of the comparison between the transmittingend high priority sequence number [H+0] and the receiving end highpriority sequence number [H+0], since the values of these high prioritysequence numbers are equal, no transmission instruction is performed. Onthe other hand, as the result of the comparison between the transmittingend high priority sequence number [H+3] and the receiving end highpriority sequence number [H+1], since the values of these high prioritysequence numbers are not equal, a retransmission instruction isperformed. In this case, the values ranging from the value obtained byadding 1 to the value of the receiving end high priority sequence number[H+1] to the value of the transmitting end high priority sequence number[H+3], i.e., “H+1” and “H+3”, are output to the retransmissioninstruction output unit 26 as the values of the high priority sequencenumbers user for the retransmission instruction. At this time, in thehigh priority sequence, number management unit 91, the value of thestored receiving end high priority sequence number is incremented twiceto be “H+3”.

On receipt of “H+2” and “H+3” as the values of the high prioritysequence numbers, the retransmission instruction output unit 26 outputsa retransmission request with the high priority sequence number [H+2]and a retransmission request with the high priority sequence number[H+3], to the transmitting end.

Then, in the data transmission apparatus 109 at the transmitting end,the retransmission requests are received by the retransmissioninstruction receiving unit 14, and the management information in thesequence number correspondence management unit 82 is retrieved on thebasis of the requested high priority sequence numbers [H+2] and [H+3],thereby obtaining the sequence number [S+1] corresponding to the highpriority sequence number [H+2] and the sequence number [S+2]corresponding to the high priority sequence number [H+3].

Further, in the retransmission decision unit 16 c, it is decided whetherthe data of the packets corresponding to the sequence numbers [S+1] and[S+2] are stored in the retransmission buffer 17 or not. Based on theresult of this decision, only the packets the data of which are storedin the retransmission buffer 17 are output as retransmission packetsfrom the retransmission buffer 17 to the transmission queue managementunit 12. Here, the high priority packets (S+1) and (S+2) are output asretransmission packets.

In the retransmission queue management unit 12, the transmission orderis set for these retransmission packets, and these packets areretransmitted to the receiving end through the transmission unit 13.Since the retransmission packets (S+1) and (S+2) are high prioritypackets, when transmitting these packets the values of theirtransmitting end high priority sequence numbers stored in the highpriority sequence number management unit 81 are incremented.

To be specific, when transmitting the retransmission packet (S+1), thevalue of the transmitting end high priority sequence number [H+3] storedin the high sequence number management unit 81 is incremented to “H+4”,and the sequence number [S+1] of the retransmission packet (S+1) and thetransmitting end high priority sequence number [H+4] are entered byone-to-one correspondence, in the sequence number correspondencemanagement unit 82.

Further, when transmitting the retransmission packet (S+2) the value ofthe transmitting end high priority sequence number [H+4] stored in thehigh priority sequence number management unit 81 is incremented to“H+5”, and the sequence number [S+2] of the retransmission packet (S+2)and the transmitting end high priority sequence number [H+5] areentered, by one-to-one correspondence, in the sequence numbercorrespondence management unit 82.

As described above, according to the ninth embodiment of the invention,data transmission from the transmitting end to the receiving end iscontinuously performed in units of packets, each packet havingadditional information relating to its sequence number, priority, anddata reproduction time, and information relating to the high prioritysequence number managed at the transmitting end and, simultaneously,data of received packets are successively reproduced at the receivingend. The value of the transmitting end high priority sequence numberpossessed by the received packet (number of transmitted high prioritypackets is compared with the value of the receiving end high prioritysequence number, managed at the receiving end (number of received highpriority packets), and a retransmission request is made by indicating atransmitting end high priority sequence number which is absent.Therefore, the transmission quality of the ratio section in real-timetransmission can be improved by retransmission of error packets thepriorities of which are equal to or higher than a predetermined valueand, moreover, the retransmission of error packets can be realized bysimpler procedures.

In this ninth embodiment, when the high priority sequence numberscorresponding to plural high priority packets transmitter arecontinuously absent, the receiving end sends a retransmission requestfor each high priority packet having the absent high priority sequencenumber, to the transmitting end. However, retransmission requests forplural high priority packets may be sent collectively to thetransmitting end, by listing the values of the plural high prioritysequence numbers or indicating the range of these values.

In this case, at the transmitting end; based on the plural high prioritysequence numbers requested from the receiving end, the sequence numberscorresponding to the respective transmitting end high priority packetsare obtained by retrieval, and the high priority packets having thesequence numbers so obtained are retransmitted to the receiving end.

[Embodiment 10]

In a data transmission method according to a tenth embodiment of thepresent invention, the number of times of retransmission requests to thetransmitting end and the interval of the retransmission requests arechanged according to the transmission status of the radio section,whereby the probability of normal transmission of the retransmissionrequests to the transmitting end is increased to improve thetransmission quality in the radio section.

FIG. 32 is a block diagram illustrating a data transmission apparatus110 in a data transmission system performing real-time data transmissionaccording to the data transmission method of this tenth embodiment.

The data transmission apparatus 110 includes a retransmission decisionunit 16 d, instead of the retransmission decision unit 16 c of the datatransmission apparatus 109 of the ninth embodiment. The retransmissiondecision unit 16 d performed the same process as that of the decisionunit 16 c and, further, outputs the sequence number of the packet whichis decided to be transmitted. Further, the data transmission apparatus110 includes a sequence number correspondence management unit 82 a,instead of the sequence number correspondence management unit 82 of thedata transmission apparatus 109. The sequence number correspondencemanagement unit 82 a performs the same process as that of the unit 82and, further, deletes the value of the transmitting end high prioritysequence number corresponding to the value of the sequence numbersupplied from the retransmission decision unit 16 d. Other constitutesof the data transmission apparatus 110 of this tenth embodiment areidentical to those of the data transmission apparatus 109 of the ninthembodiment.

FIG. 33 is a block diagram illustrating a data receiving apparatus 210in the data transmission system performing real-time data transmissionaccording to the data transmission method of this tenth embodiment.

The data receiving apparatus 210 includes an error packet detection unit22 b, instead of the error packet detection unit 22 a of the datareceiving apparatus 209 of the ninth embodiment The error packetdetection unit 22 b performs the same process a that of the unit 22 aand, further, decides the transmission status of the radio section, fromthe number of the detected error packets.

Further, the data receiving apparatus 210 includes a retransmissioninstruction output unit 26 d, instead of the retransmission instructionoutput unit 26 c of the data receiving apparatus 209. The retransmissioninstruction output unit 26 d performs the same process as that of theunit 26 c and, further, outputs the transmitted retransmission requestas a control signal.

Further, the data receiving apparatus 210 includes a retransmissioninstruction consecutive output unit 93 which receives the control signal(retransmission request) output from the retransmission instructionoutput unit 26 d, and consecutively outputs the retransmission requestby a predetermined number of times at predetermined intervals. Further,the unit 93 changes the number of output times of retransmission requestand the output interval, according to information indicating thetransmission status of the radio section, which is output from the errorpacket detection unit 22 b.

Other constituents of the data receiving apparatus 210 of this tenthembodiment are identical to those of the data receiving apparatus 209 ofthe ninth embodiment.

Next, the function and effect will be described.

Since the fundamental data transmission process according the datatransmission method of this tenth embodiment is identical to that of theninth embodiment, only a distinctive process of this tenth embodimentwill be described hereinafter.

In this tenth embodiment, the packet from the transmitting end isreceived by the receiving unit 21 and output to the error packetdetection unit 22 b. In the error packet detection unit 22 b, thetransmission status of the radio section is detected according to theoutput of the receiving unit 21, and information indicating thetransmission status of the radio section is supplied to theretransmission instruction consecutive output unit 93.

On the other hand, when the high priority sequence number output fromthe retransmission sequence output unit 92 is input to theretransmission instruction output unit 26 b, a retransmission requestindicating the high priority sequence number is output from theretransmission instruction output unit 26 b to the transmitting end and,simultaneously, this retransmission request is output as a controlsignal to the retransmission instruction consecutive output unit 93.

Then, the retransmission instruction consecutive output unit 93 performsa consecutive retransmission process for consecutively transmitting theretransmission request by several times. In this consecutiveretransmission process, the number of transmission times of theretransmission request and the transmission interval are adjusted on thebasis of a predetermined value, according to the transmission status ofthe radio section which is obtained from the output information of theerror packet detection unit 22 b.

For example, when many transmission errors occur in the radio section,the number of request transmission times is increased and the outputinterval is increased. Thereby, the probability of normal transmissionof the retransmission request to the transmitting end increases. On theother hand, when not many transmission errors occur in the radiosection, the number of request transmission times is decreased, and theoutput interval is narrowed. Thereby the time required forretransmission is reduced.

Further, at the transmitting end, packet retransmission is carried outaccording to the retransmission request, and the sequence numbercorrespondence management unit 82 a deletes the value of thetransmitting end high priority sequence number corresponding to thesequence number supplied from the retransmission decision unit 16 d.

Therefore, with respect to the same retransmission request which isreceived again, retrieval for the sequence number corresponding to thisretransmission request by the retransmission decision unit 16 d ends infailure because the correspondence between the transmitting end highpriority sequence number indicated by this retransmission request andthe sequence number is deleted from the sequence number correspondencemanagement unit 82 a.

As the result, in the retransmission decision unit 16 d, it is decidedthat no retransmission is performed for the high priority packet forwhich the retransmission request has been made by indicating the valueof the transmitting end high priority sequence number. Therefore, at thetransmitting end, it is avoided that the same packet is repeatedlyretransmitted when the same retransmission request is output severaltimes from the receiving end.

As described above, according to the tenth embodiment of the presentinvention, a retransmission request indicating the high prioritysequence number of a desired packet is consecutively transmitted severaltimes, from the receiving end to the transmitting end, againsttransmission errors. Therefore, when at least one of the severaltransmission requests from the receiving end is normally received at thetransmitting end, the error packet the priority of which is equal to orhigher than a predetermined value can be retransmitted, whereby thetransmission quality in the radio section in real-time transmission canbe effectively improved.

Hereinafter, a description will be given of the data structure of apacket Pa for transmitting data by a data transmitting method accordingto any of the aforementioned embodiments.

FIGS. 34(a)-34(c) are diagrams illustrating the data structure of thepacket Pa.

This packet Pa is composed of a header section Ph which containsrelevant information indicating the attribute of the packet, and a datasection Pd which stores data to be transmitted (FIG. 34(a)).

The header section Ph includes header information Ia indicating thesequence number corresponding to each packet, header information Ibindicating the reproduction time at the receiving end, of the data to betransmitted (time stamp) Ib, header information indicating the priorityof each packet, extension header information Id, and other headerinformation I1 to I10 (refer to FIG. 34(b)).

The specific convention of each header information is described inRFC1889 as shown in FIG. 34(c). For example, the header information I3indicates that the extension header information Id is added to theheader section Ph when its value x is 1. The header information I5indicates that the data stored in the data section is coded data by theMPEG1 method when its value PT is 32, and indicates that the data storedin the data section is coded data by the MPEG2 method when PT is 33.Further, each of the header information I9, Ic, I10, and I11 is headerinformation to be added when the MPEG1 coded data is transmitted by RTP.The value P (P=1) of the header information Ic indicates that the datain the data section is I frame data, and the packet containing this Iframe data is to be treated as a high priority packet. The value P (P=2)indicates that the data in the data section is P frame data, and thepacket containing this P frame data is to be treated as a low prioritypacket. The value P(P=3) indicates that the data in the data section isB frame data, and the packet containing this B frame data is to betreated as a low priority packet.

Further, the extension header information Id corresponds to the sequencenumber and the priority information of the previous packet in the thirdembodiment (refer to FIG. 6), the sequence number and the retransmissioncount of the previously transmitted high priority packet in the fourthembodiment (refer to FIGS. 13 and 14), the sequence number and thereproduction time of the previous packet in the second modification ofthe sixth embodiment (refer to FIG. 21), the difference value of thesequence number of the previous packet and the difference value of thereproduction time of the previous packet in the third modification ofthe sixth embodiment (refer to FIG. 22), and the transmitting end highpriority sequence number in the ninth embodiment (refer to FIG. 29).

1. A data transmission method for transmitting data in units of packets,said method comprising: a first step of performing transmission of thepackets each having a sequence number; a second step of receiving aretransmission request for a retransmission packet corresponding to anyof the packets in which a transmission error has occurred; and a thirdstep of performing transmission of the retransmission packet having afirst sequence number indicating a reproduction order and a secondsequence number indicating a transmission order of the retransmissionpacket, on the basis of the retransmission request.
 2. The datatransmission method of claim 1, wherein the retransmission packet is ahigh priority packet.
 3. The data transmission method of claim 1,wherein the second sequence number assigned to the retransmission packetis included in an RTP header.
 4. The data transmission method of claim1, wherein the second sequence number of a next retransmission packetwhich is to be transmitted next to the retransmission packet isincremented by one from the second sequence number of the retransmissionpacket.
 5. The data transmission method of claim 1, wherein the thirdstep is performed only when the retransmission packet is a high prioritypacket.
 6. The data transmission method of claim 1, wherein the secondstep is performed in a case where a time at which a data receivingapparatus receives the retransmission packet is prior to an arrival timelimit of the retransmission packet.
 7. A data transmission method forreceiving data in units of packets, said method including when atransmission error occurs in any of the packets each having a sequencenumber, a first step of transmitting a retransmission request for aretransmission packet corresponding to the packet in which thetransmission error has occurred; and a second step of receiving theretransmission packet having a first sequence number indicating areproduction order and a second sequence number indicating atransmission order of the retransmission packet, which has beentransmitted from a data transmission apparatus on the basis of theretransmission request.
 8. The data transmission method of claim 7,wherein the retransmission packet is a high priority packet.
 9. The datatransmission method of claim 7, wherein the second sequence numberassigned to the retransmission packet is included in an RTP header. 10.The data transmission method of claim 7, wherein the second sequencenumber of a next retransmission packet which is to be transmitted nextto the retransmission packet from the data transmission apparatus isincremented by one from the second sequence number of the retransmissionpacket.
 11. The data transmission method of claim 7, wherein the secondstep is performed only when the retransmission packet is a high prioritypacket.
 12. The data transmission method of claim 7, wherein the firststep is performed in a case where at a time at which a data receivingapparatus receives the retransmission packet is prior to an arrival timelimit of the retransmission packet.
 13. A data transmission apparatusfor transmitting data in units of packets, said apparatus comprising: atransmission unit for performing transmission of the packets each havinga sequence number; and a retransmission instruction receiving unit forreceiving a retransmission request for a retransmission packetcorresponding to any of the packets in which a transmission error hasoccurred; wherein the transmission unit performs transmission of theretransmission packet having a first sequence number indicating areproduction order and a second sequence number indicating atransmission order of the retransmission packet, on the basis of theretransmission request.
 14. The data transmission apparatus of claim 13,wherein the retransmission packet is a high priority packet.
 15. Thedata transmission apparatus of claim 13, wherein the second sequencenumber assigned to the retransmission packet is included in an RTPheader.
 16. The data transmission apparatus of claim 13, wherein thesecond sequence number of a next retransmission packet which is to betransmitted next to the retransmission packet is incremented by one fromthe second sequence number of the retransmission packet.
 17. The datatransmission apparatus of claim 13, wherein the transmission unittransmits the retransmission packet only when the retransmission packetis a high priority packet.
 18. The data transmission apparatus of claim13, wherein the retransmission instruction receiving unit receives theretransmission request in a case where a time at which a data receivingapparatus receives the retransmission packet is prior to an arrival timelimit of the retransmission packet.
 19. A data transmission apparatusfor receiving data in units of packets, including a retransmissioninstruction output unit and a receiving unit, wherein when atransmission error occurs in any of the packets each having a sequencenumber, said retransmission request instruction output unit transmits aretransmission request for a retransmission packet corresponding to thepacket in which the transmission error has occurred; and said receivingunit performs reception of the retransmission packet having a firstsequence number indicating a reproduction order and a second sequencenumber indicating a transmission order of the retransmission packet,which has been transmitted from a data transmission apparatus on thebasis of the retransmission request.
 20. The data receiving apparatus ofclaim 19, wherein the retransmission packet is a high priority packet.21. The data receiving apparatus of claim 19, wherein the secondsequence number assigned to the retransmission packet is included in anRTP header.
 22. The data receiving apparatus of claim 19, wherein thesecond sequence number of a next retransmission packet which is to betransmitted next to the retransmission packet from the data transmissionapparatus is incremented by one from the second sequence number of theretransmission packet.
 23. The data receiving apparatus of claim 19,wherein the receiving unit recieves the retransmission packet only whenthe retransmission packet is a high priority packet.
 24. The datatransmission apparatus of claim 19, wherein the retransmissioninstruction output unit transmits the retransmission request in a casewhere a time at which the data receiving apparatus receives theretransmission packet is prior to an arrival time limit of theretransmission packet.